Bands? Talkin' bands reninds me of talkin' bow wood.
Theraband is, imo, best exercise band for slings. I don't think WM sells them. Brand WM sells may work ok, though. Different band colors signify different pull weight. In Theraband world, gold color is king.
Again imo, medical-grade latex (.030 or .040) is pinnacle of ss flat banding. Superior retraction rate = fast. But their life is bit limited & ya gotta keep them in cool/dark place.
For lonnnnnng life, I've not found anything that can hold candle to this stuff ... simple-shot.com/diy/linatex-sheets/
For these bands my typical band setup (for 3/8 steel) is 'bout 5 1/2" long working length x 7/16 wide, straight (no taper) cut.
For latex or theraband, my recollection is either 4 or 5 stretch factor. If draw is 30", working length of bands would be approx 6".
This is my go-to forum for anything related to slingshots ...
http://slingshotforum.com/ Wonderful helpful folk, just like PA!
Here'c couple vids I shared today with some friends.
https://www.youtube.com/watch?v=q2lIXEY_xichttps://www.youtube.com/watch?v=6fJ__Cb9eIs