Invitation Envelope size

I had always planned on a 5x7 invitation and getting A7 envelopes. I am having a hard time finding a website though that carries an outer envelope for the A7's. I really thought there was a A7.5 or an A8.

Has anyone used this size or do you think I should just go to A9?

Suggestions or advice would be immensely appreciated.

TIA!

Hi, I just ordered some envelopes from paperandmore.com. they have a great variety of colors and textures.
I'm making the invitations myself and using A9 envelopes, which is a regular size paper folded in half.
good luck!