Pretty easy if your are building and HD or XS or such with a 16" rear wheel. Avon, Dunlop, Coker, Maxxis, Metzeler, Continental. 130mm and wider. For 18" I know you can go with Avon or Maxxis.
Prices range from $70 and up in USD. Coker, Maxxis and Avaon are damned soft, don't last very long. My friend had an Avon Gangster on his 1500 Drifter, tire was done in less than 10 thousand miles. A Dunlop or Continental should rot away before the tread wears out, I had over 40 thousand miles on a rear 150/80/16 D404 on my 800A Vulcan a few years back and it had at least 60% tread.