I think I am going to replace my USPF 45 barrel at around 30k rounds.  Where can I find new ones?  Maybe Match barrels?

Thanks

Try Nowlin, maybe Bar-Sto? Or do you mean H&K factory barrels?

ccfa.com
advancedarmament.com

these are the 2 I thought of off the top.they also supply suppressors.

You can replace it if you want to but I can tell you this, unless you have neglected your barrel at 30K that barrel is just getting broken in.  Put it this way, the Expert comes with 5 o-rings for the barrel.  HK predicts the life of an o-ring when given good care to last for 20,000 rounds.  An o-ring takes a lot more wear and tear than the inside of a barrel.
Replace it if you want for any reason you want, but if you are replacing it because of round count alone you are replacing a perfectly good barrel.  

I basically wanted to replace it with a match barrel so I could compare the accuracy with the standard one.  I would never throw away a barrel that shoots through the same hole at 10 yards.  I was just looking for suppliers besides the HK factory itself.

I think it would be hard to improve on the stock item but Jarvis and AAC and Barsto are all great recommendations i have had great service from all three.

I am fairly certain BAR-STO is out of the HK barrel making business.  The best reports that I have heard come from Jarvis and CCF barrels and both come with the o-ring cut if you get the full size barrels.  FYI Jarvis cuts the o-ring on Compact barrels also but CCF does not.
Another company, EFK, does HK barrels and I have not heard of any bad reports on their barrels.  I have used a bit of their 1911 stuff and have been completely satisfied with it but I have not heard all that much about their HK barrels, although what I have heard has been positive.  EFK does not cut the o-ring in any of their barrels.
All can be had in the extended 6 inch barrel and/or extended and threaded.  Just make sure if you are getting the 6 inch barrel and the o-ring cut that you specifiy that the cut is for the Match pistol (uses the standard USP slide as does the Tacitical) so you don't end up getting the o-ring groove cut in the wrong spot.  The o-ring location is different for the USP (which includes Tac and Match) and the Expert and Elite, which both have longer and different length slides.