How did you source for Wine & Beer?

Our catering contract states that we will have a bartender and able to serve alcohol, but we must supply our own.

For those with similar setup, how did you source your wine and beer? We are also doing his&hers cocktails and will need to get things for that.

Our wedding is in October and we're estimating around 80-100 guests! Been looking at Costco and other places, but is there something I'm missing? Do I just buy a shit ton of alcohol? I'm worried about running out!

Any advice would be helpful! Thank you!