A single 12L is just a single 12L, not much DIR to it, and it's by far the most common cylinder over here. Your local diveshop should have a pile of them. I prefer the concave bottom variant for singles as it can stand up without a boot.
If you're looking for doubles, they possibly may not have them in stock (but they would certainly order). There are usually a lot on the second hand market as well. Double 12s are common for tech divers and many seem to migrate to RBs and their twinsets become available.
For new, I like to order myself and get my cylinders from a couple of German dive shops:
www.deepstop.de
SF-1 ScubaFreaks Steel cylinder without valve, steel cylinders up to 5 liters, steel cylinders 7 liters, steel cylinders 8,5 liters, steel cylinders 10 lite
www.sf-1.eu