Theoretically, you would go to a local, independent agent, and buy a special event policy from them with liquor liability. It's probably going to cost you between $500 and $750 - for one day. And, you're going to have a REAL hard time finding someone to sell you one, if you don't have the certifications, or if you don't have general liability coverage also.
On the other hand, if she - or your parents, if they are hosting the wedding - may have automatic coverage under their homeowners or renters insurance, if it's not a cash bar. OR, it might cost them $100 extra. If it's not a cash bar.
If it's a cash bar, SOMEONE is going to have to pay for the liquor liability and general liability related to the bar.