I believe that Patrick's Fly Shop carries both 30 and 50-pound spools of Cortland braided mono; probably others as well since we have a lot of line builders in the Puget Sound country. It is about $12 for a 100 foot spool as I recall. I use 30-pound for 4 through 8-weight lines; 50-pound for 9 through 13 weight lines. The stuff is nearly indestructable if you install it properly.
Some of my Baja fishing pals also use Gudebrod 35 or 50-pound braid which they like. I don't recall where they purchase it.
The answer depends on your application. putting a loop at the end of a tapered trout line? 30 lb is great. putting loops at the ends of a lead core or other level head, or putting loops into the belly of a line to build a multi-tip? I'd use 50.