So, first off - I'm pretty inexperienced with cars. I'm looking to buy a new muffler for my bonneville and I know that I need a series 80 muffler since I don't have dual exhaust. But I'm a little confused... Are all series 80 mufflers the same size? And if they are can I just go and buy any series 80 muffler and replace my old one?

Most guys on here have gone with either a magnaflow or a flowmaster brand. But whatever shop you go to will handle all sizing and fitting issues. Hope ur ready to make some noise.LOL

Alright but what if I wanted to order one and put it on by myself?

Im assuming ur buying online. I think these mufflers are relatively universal in some aspects. But any online aftermarkets retailer usually asks for the year make and model of you car before listing parts available for that car.

Flowmaster Exhaust Systems, Mufflers, and Exhaust Accessories - JEGS

The Official Site of MagnaFlow Performance Exhaust - Exhaust Systems For Trucks, Suv'*, American Muscle, Diesel, & Sport Compact Vehicles

So once you decide where u want to buy it from it should'nt be hard match one up to ur ride. Im curious though, u said ur not real experienced with cars, but u want to install ur own muffler? Im the last one to discourage ambition and getting ones hands dirty, but i wasn't expecting that response from u.lol

Lol well you gotta start somewhere :p