Walk into any pharmacy and tell them what you want, no prescription necessary.  Now, depending on what you bought, there may be an issue with returning to the US.  If you have a RX, no problem, but without one, you may just want to not mention the medication, as long as it isn't a class one drug (I think class one is along the lines of Valium, Xanac, etc.)  I have crossed many times with viagra, celebrex and other basically OTC drugs and not had a problem.  Maybe I have just been lucky, but you can contact the INS and ask what the procedure is for re-entering with the above type drugs.