I found muriatic (31% I think) at Lowes and home depot- in the outdoor section (next to flagstones and such)
Often called concrete etchant or cleaner (is used to remove oil stains from driveways!)
Lowes sales 1 gallon bottles for $7-8; Home Depot sales the same bottles, but in a cardboard box with 2 bottles for $10; if you're gonna use much, Home Depot much better deal but not worth the storage hassles if not using much, so buy the single bottles for slightly more IMO...
Ace may have it in smaller sizes, but probably higher cost...
Many other places may have it, as stated above, as driveway cleaner...
Good Luck!