I believe they are handed to you by your airline - check your airlines website - if not go to your airports website and there should be info on there. On that basis you should be able to get the size and buy some at your local supermarket
Try stationery shops the maximum size the bag can be I think is about half the size if an A4 piece of paper (folded down) and must be sealable, that's what it says on the Manchester Airport website. Some supermarkets may also stock these bags too.
The bags given at Manchester airport are a lot smaller than the ones we were given at MCO to come home. The same rule applies that you cannot take anything bigger than 100ml.