Here is a source that the American Philatelic Society recommended to its readers, since it no longer carries clear mounts.
I have placed one order with ClearBags, and the experience was a very positive one. If you visit their website, you may be overwhelmed by the number of choices. My order was for 5 different sized envelopes in single stamp to large cover format. This saved on postage.
If I remember correctly, you can order envelopes/bags without adhesive mounting strips.