I've compiled a list of the top Spark accessories for May 2019. These accessories were chosen based on their sales volume on Amazon over the past month. If you're looking for some new accessories to add to your kit, this list will give you an idea of what other Spark owners are buying and using.
DJI Spark Battery
Notable features:

SKYREAT Foldable Tablet/Phone Mount
Notable features:

Threeking Parabolic Signal Booster
Notable features:

Fstop Labs Foldable Tablet Mount
Notable features:

DACCKIT Carrying Case
Notable features:

AxPower OTG Cable
Notable features:

KINBON Landing Pad
Notable features:

Kuuqa Accessories Kit
Notable features:

LEOBRO Accessory Kit
Notable features:

SKYREAT Foldable Tablet/Phone Mount (New Design)
Notable features:

Deyard Carrying Case
Notable features:

DJI Spark Prop Guard
Notable features:

RCstyle Parallel Battery Charger
Notable features:

RCGEEK Foldable Tablet/Phone Mount
Notable features:

SKYREAT Extended Tablet/Phone Mount
Notable features:

PGYTECH Foldable Tablet/Phone Mount
Notable features:

Yx Parallel Battery Charger
Notable features:

RCGEEK Battery Charger
Notable features:

Hermitshell Remote Controller Case
Notable features:

Fstop Labs Extended Landing Gear
Notable features:

DJI Spark Battery
Notable features:
- Created, designed, and extensively tested by DJI
- Guaranteed to work well with your Spark since the battery is an OEM DJI accessory
SKYREAT Foldable Tablet/Phone Mount
Notable features:
- Made of aviation aluminum-alloy
- Able hold a mobile device between the width of 4-12 inches
- Mount can be swiveled sideways to hold a phone
- Hole at base to attach a lanyard
Threeking Parabolic Signal Booster
Notable features:
- Boosts the remote controller signal to increase its range
- Folds closed for storage
Fstop Labs Foldable Tablet Mount
Notable features:
- Made of high impact ABS plastic
- Attach and detach within seconds
- Able hold a mobile device between the width of 4-12 inches
- 360 ball joint allows the tablet to be adjusted for better visibility
- Folds into a compact form factor for storage
- Hole at base to attach the included lanyard
DACCKIT Carrying Case
Notable features:
- Tough and durable hard outer shell
- Carry with the adjustable shoulder strap or side carry handle
- Can hold the Spark, remote controller, 5 batteries, charger, charging hub, and other small accessories
AxPower OTG Cable
Notable features:
- Connect mobile device to the Micro USB port on the bottom of the remote controller
- Allows Wi-Fi to be disabled on mobile device to prevent possible interference
KINBON Landing Pad
Notable features:
- 30 inches wide
- Made of a waterproof nylon material
- Includes stakes to ensure the landing pad stays in place
- Fold up and store in the included storage bag
Kuuqa Accessories Kit
Notable features:
- Protect the remote controller sticks while in storage with the remote controller stick protector
- Lock the gimbal in place and protect the camera with the gimbal lock and cover
- Protect the props when in flight with the prop guard
- Shield the camera from the sun and protect the camera with the lens hood
- Protect your fingers when hand catching with the finger guard
LEOBRO Accessory Kit
Notable features:
- Lock the gimbal in place and protect the camera with the gimbal lock and cover
- Protect the remote controller sticks while in storage with the remote controller stick protector
SKYREAT Foldable Tablet/Phone Mount (New Design)
Notable features:
- Made of aviation aluminum-alloy
- Able hold a mobile device between the width of 4-12 inches
- Mount can be swiveled sideways to hold a phone
- Hole at base to attach the included lanyard
- This upgraded version is more durable since the plastic tightening ring has been replace with a metal ring
- Store the mount and lanyard in the included storage bag
Deyard Carrying Case
Notable features:
- Portable and lightweight
- Hard outer shell to protect the contents
- Can hold the Spark, remote controller, 3 batteries, charger, and other small accessories
DJI Spark Prop Guard
Notable features:
- Prevent the props from making contact with people or objects mid-flight
- Created, designed, and extensively tested by DJI
- Guaranteed to work well with your Spark since the prop guard is an OEM DJI accessory
RCstyle Parallel Battery Charger
Notable features:
- Charges 4 batteries simultaneously in about 60 minutes
- Able to also charge 2 USB devices
- Can also be powered via a car cigarette lighter charger
RCGEEK Foldable Tablet/Phone Mount
Notable features:
- Made of aluminum alloy
- Able to hold a phone or tablet
- Adjustable to customize viewing angle
- Foldable design for convenient storage
- Hole at base to attach a lanyard
SKYREAT Extended Tablet/Phone Mount
Notable features:
- Able hold a mobile device between the width of 4-12 inches
- Mount can be swiveled sideways to hold a phone
- Adjustable height and position
PGYTECH Foldable Tablet/Phone Mount
Notable features:
- Able hold a mobile device between the width of 4-10.5 inches
- Made of aviation CNC aluminum
- Mount can be swiveled sideways to hold a phone
- Foldable design for convenient storage
Yx Parallel Battery Charger
Notable features:
- Charges 4 batteries simultaneously in about 60 minutes
- Able to also charge 2 USB devices
RCGEEK Battery Charger
Notable features:
- Charges 1 battery in 90 minutes
- Can also be powered via a car cigarette lighter charger
Hermitshell Remote Controller Case
Notable features:
- Hard outer shell
- Keep the sticks attached while transporting or storing the remote controller
- Protects the remote controller and prevents it from being scratched
Fstop Labs Extended Landing Gear
Notable features:
- Made of high impact ABS plastic
- Extends the landing gear by 1.5 inches
- Wider stance makes landings more stable
- Easy to snap on and remove