Tons of online choices, I try to keep my money as local as possible, so I purchase mine from a locally owned print shop.

CFS Taxtools has supplies tailored to work with Intuit software; I've ordered from them for decades. Their prices are very competitive and they are a great company to work with.  Customer service is outstanding.  (Intuit, hear that????).

They also have several software offerings that are useful (@George4tacks calls 'em the 'swiss army knife for tax preparers).

Most clients do not care about the fanciness of a folder. Why don't you try getting some regular 10x14 manila envelopes, high quality. Stamp your name, address, phone #, on it and confidential and important. Then you can print the client's name on there. This is simple quick and easy.

Tenenz sells tax and accounting products. They have a new name but that should still get you to them