You will be hard-pressed to find good quality SA keycap sets, let alone SA sets that are regularly stocked or widely available.
My first recommendation will be that you peruse what is available at
https://pimpmykeyboard.com/ - this is Signature Plastic's website. These are the OG creators and makers of the SA profile and its sibling profiles (SA-P, DSA to name a couple). I can tell you that Signature Plastic tooling and ABS is top-notch quality in my personal SA Bliss set. Super thick and thocky.
Secondly, you can check out
https://www.originativeco.com/They carry a few SA and DSA keysets, but primarily focus on GMK and BSP.
For the rest, you can check out the SA keysets on MechanicalKeyboards.com, KBDfans, Aliexpress, and KRepublic; join a groupbuy of an SA set; or find the SA kit of your desire (at a premium) either at r/mechmarket or here in the Classifieds.
NOTE: Outside of Signature Plastics and Keyreative SA sets, the rest are on the thinner side, but do come at a range of colors that are readily available.