ask for NitrousPlus or Supernitrous, that is nitrous oxide with sulfur in it ( thats to prevent people from sniffing it to get high)
Or jsut explain to them that you want to buy nitrous oxide for use in a race car, usually 64 or 54lb mother bottles. Also be sure to find out if the have dip tube or not, If you have a choice get the one with NO diptube. Also get in touch with Monte Smith or someone, you will need the stand that hold the mother bottle, braided lines and bottle nuts and possibly nitrous in line filter to hook up the bottles. Good luck