I'll second McMaster-Carr as a source. Great selection, pretty decent prices, and availability in a very large selection of thicknesses, much more than you'd normally see. 6 or 8 mil is significantly more durable than the standard size bags, making them more reusable, too.
In addition to the normal zip lock types I also picked up some corrosion resistant bags and bubblewrap type bags inexpensively and easily as I happen to live near a McM location. Their catalog is literally like a thick brick,tons of interesting stuff, and they sell to anyone unlike some other industrial suppliers.
Staples now stocks a selection of resealable bags in and around that size, too, from what I saw today. I don't know how thick they are, though.
Uline might be another choice as they have all sorts of stuff like this, and a great selection of paper boxes, too.
eBay always has vendors selling bags of different sizes and style, and usually at great prices, too.