There are plenty of people that can build quality strings as to the material BCY 452X, 450 plus, and 8125 would be my choices. To buy commercial strings in these materials vapor trail and winners choice are about the best options to go custom there are a bunch of places to get them.
I like H&M in PA http://www.hmbowstrings.com/
but Mike usually runs 4-6 weeks due to being backed up all the time
Jamie at Twisted Archer makes good strings and usually has them built in 2 days. He is currently running a deal on any set of strings for $50 but his normal price is only $54 for most bows
Mike makes good strings with fast turn around for $45 a set