First of all, note that your Alpha Black uses the same barrel threads as a Tippmann Model 98 or 98 Custom.
Custom Products make a very good one-piece barrel.
I wouldn't expect huge accuracy gains from changing your barrel, though, and I certainly wouldn't advise spending more than $30 or so on one.
Also, be wary of getting too long a barrel and making the gun too unwieldy to carry. Anything over 14" makes a Tippmann preposterously long.