Wholesale Parts and Film in Readyville, Tennesee, USA lists a bunch of it on their website. I am sure there are plenty of others.
I picked up a 500 ft spool of 30 ga for $1 at a junk sale.
There should be information available on the resistance per unit length for various diameters of wire. You can then determine how much wire you need. Note that if you intend for the wire to get very hot then the resistance will change with temperature. My guess is that the manufacturer's might have the data you need and possibly some applications information.