ha HA! this happened to my boyfriend and me the first time we had sex. go to planned parenthood. if you do a google.com search, you'll be sure to find the website, and from there you can find one near you. if you can't find a PP near you, then go to a doctor. morning after pills are prescription and so you cant just get them in a grocery store or something. good luck.
quick edit: how long has it been since your girlfriend and you did the mattress mambo? if it's been more than five days, don't even bother. it's preferrable to get a prescription for morning after pills 72 hours after intercourse, but sometimes it takes close to five days.