Check out Regimental Quartermaster out of Gettysburg, https://regtqm.com/home/ They have a wide range of products from lower end starter stuff to higher end campaigner quality stuff from some of the best out there. S&S, also out of Gettysburg are good too, https://www.ss-sutler.com/
Agreed with the others. Definitely avoid buying uniforms from them. They are very over-priced and accuracy is questionable at best. I think the Quartermaster shop makes their uniforms for them. The civilian clothes are OK, but, definitely run as far as you can from the military stuff!
Also, always check with your unit to see whom they recommend. There are a number of regional vendors that do as well as the 'big guys' for a bit better price, and its possible they even give your unit's members a slight discount. My rule of thumb... if your mates aren't familiar with a particular vendor, but curiosity prompts you to buy from them, start with something small and then compare quality with some of the more established brands.