You got one of two choices:
Take it to Spectrum Gas Products in Costa Mesa or Take it to General Joe's in Azusa, CA. and they can courier(sp?) it to Spectrum Gas Products in Costa Mesa. Either way I think the guys at Spectrum Gas Products are great guys and you should get your tank in a day or two...I can't remember how much...but it was defintely pretty cheap compared to other places that I took my tanks to.
Spectrum Info: http://www.spectrumgas.com/
General Joe's Paintball: http://www.generaljoes.com/storeHours.html
A.Oribello "Big Nasty"
Splat Factory Indoor Paintball (Brea, CA.) A great place to get your paintball FIX!