spinoff to inner envelopes (HELP!)

Do inner envelopes come in most sizes or only one? I have 5 1/2 x 8 1/8 (size A8) and I assumed that I could just get the others when I needed them, but now on paperpresentation.com, I see that A7 has a special size for OUTER not for INNER...

Did I mess up thinking that I could get inner envelopes for whatever size I needed???

Help