For example, here they're selling 500 standard business envelopes for less than $18:
500 - #10 Regular Envelopes, 24lb White Wove Paper, Gummed Flap
The maddening thing about Amazon, of course, is that if you enter "white envelopes made in usa", 7 of the first 10 results were made in China! And while one of this company's products shows up, it's their 6x9 product, not their standard sized envelopes.
What's even more maddening is that the China products have so gamed Amazon search that THEY are charging a premium over products made in the USA!
Make sure to support companies like this. It's the only way to help them rise back in the search rankings where they belong.