What company would be best to make platoon t-shirts and sweatshirts? Has anyone used a company that they recommend?

I use a lot of vendors to produce RPIs as retention gifts. My favorite ones are:
Jardine
Sparta Pewter
Logo works
MP (Military Pewter) USA
Carolina Specialties International

I will tell you upfront, where most people screw up here is getting a great design and putting it on a cheap azz T-Shirt. Get a CHAMPION brand T-Shirt as a base with the embroidered C logo on it. They are excellent and last longer than the cheaper T-shirts the Army seems to prefer. They are slightly more expensive but you can get a fairly good price if you buy in bulk from a distributor.

My wife can do it. Message me if you're interested and I can connect you.