I got something a little smaller from Poundland a year or two back which was actually a pouch for a FAK. If you want something better, albeit a little larger than your dimensions, then I'd go for the Alpkit Mission packs, or at least the small one (they come in a set -
https://www.alpkit.com/products/mission ). The smallest is 15cm x 11cm, so 5.91in x 4.33in, but that would otherwise do very well. Not waterproof, but resistant, and the whole set is very useful. I've lent them to my wife when she's go on trips overseas for work, and its been very difficult to get them back off her!
Or have a look on somewhere like DX - 'zipper bag' came up with some hits like this
http://www.dx.com/p/edcgear-portabl...zer-bag-camouflage-size-s-376127#.VPXKFy6-P5w , although most of them seem to be mesh, a bit like the Eagle Creek type packing bags. Their Pack-It Spector sac is pretty similar to the Alpkit ones, but more expensive, and like most of the bags for travelling, are a bit bigger than you want. Ideally, what you want are the sort of things they give away with freebie makeup stuff on planes, or a small pencil case. Ebay might be a start, but it might take a while, because most are larger than the size you want.