I don't like using 3rd party props. If you're dead set on using them though, then the Master Airscrew props would be the brand I'd recommend.
While these props won't make the Spark super stealthy (nothing will), it will change the pitch a bit. See this video for a comparison between the OEM DJI props:
I recently purchased a set of the MS props and as stated by msinger, nothing is stealth. They are at a lower pitch that sound "quieter" but still noticeable.
Although colorful, the props don't fold like the DJI props and have to be removed if putting the Spark in a case designed for folding props.