Weddingbee boards

I had this same problem so I opted for a standard size with matching inner and outer envelopes. The only stationer I have found that offeres inner and outer #10's is Wedding Paper Divas.

http://www.weddingpaperdivas.com/wedding-invitation-envelopes.htm

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#10 envelopes measure 4 1/8 inches by 9 1/2 inches, and are used for wedding and bridal shower invitations with dimensions of 4 inches by 9 1/4 inches. You can include outer envelopes along with your #10 envelopes that will measure 4 3/8 inches by 9 3/4 inches. This outer envelope ensures that your invitations arrive at your guests' homes without postage marks on the inner envelope, and that the inner envelope flap is not gummed and thus does not need to be sealed (only the outer envelope is sealed). If you purchase liners with the double envelope option for a design from our Signature White Collection, the liners will only appear on the inner envelopes.

All of our #10 envelopes have modern square flaps and open on the long side of the envelope.

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I think you're actually looking for the outer envelope: the one that you write the address on that gets all dirty. You might not have enough room for an inner envelope.

I hope that the inner envelope you have doesn't have the flap along the short side of the envelope, because it might drive you crazy if the outer envelope has the flap in the other direction!

Also, try googling "#10 inner and outer envelopes" for other retailers.

Good luck!!