Are you missing a speaker, is the existing one dead/wrecked, or is there something about the one that's in there you don't like?
If the one that's in there is functional, do you know what it is? Brand, model, etc.
The reason I ask is people often think a new speaker will make them happier than replacing the amp. Sometimes that works out, but a lot less often than you might think if you listen too hard to the speaker manufacturers or cork sniffers. Some are happier at first, but that's usually just honeymoon happiness. Later on, they sell the amp, and give a variety of excuses. "It just didn't fit in the mix" or some similar esoteric bullshit.
All that said, I've been pretty happy with the products from WGS (
Warehouse Guitar Speakers) and Eminence. WGS makes a lot of Celestion clones, and Eminence makes the
Delta Pro 12A, which is a dead-nuts replacement for the EVM 12L, a highly-regarded guitar speaker among pro guitar and bass players both. Celestion's designs speak for themselves, but that Delta Pro is a real piece of work. I used to use the EVM 12Ls for everything I could, as they're relatively flat (for a 12" speaker) and can take more abuse then anything else on the market. You can burn up an amp easier than one of those speakers. But, they're expensive as sin and Eminence is eating their lunch on that design these days. Same specs, same sound, half the price. The only way I'd say not to use one of them is if you're counting on the speaker to do something to/for your sound. They won't. All you hear is everything from the amp back.
That's a Good Thing, by the way.