I know that the legal proffession use these to send documents, to avoid excessive folding - I would imagine most commercial stationers would sell them.
I have checked all over the net and major stationers and nobody stocks them. It's a very unconventional size but I know they exist because I have seen Marketing publications in them. And no, they don't have them at the Post Office! It's a nightmare - I have 60 invites to post out and no envelopes!
If you can't fold them the other way and use standard A5 envelopes, your cheapest option would be to use an A4 size. Maybe you could print something on them to match the invitations so that recipients know it's not junk mail.
The size normally used for A4 folded in half is the C5. The first shop that came to mind local to me has them at �3.99 for 25 white envelopes. If you have not yet succeeeded in your search, check under Office Suppliers in the Yellow Pages - they should be able to tell you which retailer stocks these if they are themselves unwilling to sell you from their stock.
This is probably too late (was distracted and forgot to check further) but the imperial size 12x5 inches is available in manilla (self seal �8.98 + VAT for 250) or white (gummed) at something like 25% more. Stationer I know of has these on tel. 01382 542168