You didn't specify Confederate or Federal. Try Richmond Depot at www.kabar44.com. They do only Confederate stuff but their work is museum quality. Their good stuff is mostly hand sewn or period machine sewn. They also do some more economical stuff. They aren't cheap but their wares are quality stuff. I've spent mucho dollars there. You will need to determine who it is that you are trying to portray in order to get outfitted correctly. A lot of cannoneers were pulled from other units to fill vacancies so not every man would have been decked out in red trimmed gear. There is C.J. Daley in Gettysburg also. Another authentic high quality dealer. Not cheap but extremely authentic. Buy good stuff if you want it to last. And remember that uniforms came in three sizes.....too small.......too big.........and don't fit. Try not to look like a fighter pilot or state trooper. The three volume set "Echoes of Glory" has one volume on clothing and equipment that is an excellent reference work.
3rd Richmond Howitzers (Smith's Battery)
Virginia Light Artillery