Amazon. Parts and pics are in that linked FM thread, but I will repeat them here.

This is the shoulder brace - there are similar ones cheaper, but I don't know if the pockets on those are the same size. So you could be taking a chance on those, but it might be worth experimenting (if you find a cheaper one that works as well or better, please reply in this thread):

https://www.amazon.com/gp/product/B073CV1NKG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

The exact shoulder pads I used (orange, Klim D30) Amazon doesn't seem to sell anymore. But these appear to be same thing (same shape and like D30, they harden on impact, which both protects the shoulder, and dissipates the impact energy). These also look more ventilated than mine, so should be even cooler. Note that they come in pairs, so a way to reduce your cost, is to go halfsies with a OW pal (also note that Amazon has a 10% off coupon on there you can use):

https://www.amazon.com/Protector-ARS-Replacement-AP-EVO-Motorcycle/dp/B07KQC396L/ref=sr_1_3?crid=3EBRHYASW4SJO&keywords=d30%2Bshoulder%2Bpads&qid=1579278897&s=automotive&sprefix=d30%2Bsh%2Cautomotive%2C183&sr=1-3&th=1

This is the velcro I cut into short strips just to extra-secure the armor in the pocket. You could also look at hot-glue/epoxy, or sewing:

https://www.amazon.com/gp/product/B0784C31L8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1