Why do you want a different speaker? What is wrong with the Blue Marvel that came in it when you bought it? I'm not sure, but that Blue Marvel may actually be a Sheffield. Peavey used Sheffields for a long time. Not bad, not great, so so, just increases their profit margin cuz they can get them cheap in volume.
What kind of music? What kind of sound? Probably not metal or you wouldn't have bought a C30. Do you like it to cut thru and be extra loud? Or do you want it a bit smoother and more controlled? Do you like more low freq or midrange in the sound? Or do you like a more cutting or piercing hi freq sound?
The thing I've heard most about Peavey amps is it is very hard to find a speaker that matches the amps sound well. I have gone thru a dozen different Eminence speakers in the last year or so, and I can tell you in no uncertain terms that the loudest and most cutting brute force Eminence speaker is by far the Tonker. Second might be the Swamp Thang. If you want crisp and dirty try the Legends 125 or V12. If you want clear and smooth and not as cutting, the smoothest Eminence imo is the Cannabis Rex. The C.Rex is like no other Eminence speaker, and I absolutely love the ones I have.
With the brighter and edgier sound of the C30, I would shy away from any Celestions. But if you must try them, try the Vintage 30 or the G12T-Hot100, both can be found on eBay for well under $100 bucks. But Celestions are extra bright and on the thin side, me thinks.
I have never been let down by Weber Speakers. The best and most versatile Weber is the Michigan, followed by the California, the Chicago, and the Blue Dog.
If you have money to burn, look at Tone Tubby first, then EV and JBL.
One thing you might do is take your amp to the guitar stores and pretend you want to buy a cab. They will most likely let you plug your amp into their cabs. If they are worth a nickel, they will be able to tell you what kind of speakers are in the cab. For example, if you want to hear Vintage 30's, try an Orange cab. Or plug the speaker cable from a Fender amp into your C30 and play thru the Fender's Eminence speakers, a lot of the current Fender amps are coming with Emi's in them. Try several different cabs, when you find one you love the sound of, find out what's in it and go buy that speaker. Just understand that the sound of one lonely speaker in your amp will never sound as good as the sound of two or four speakers in a cab, even if you buy the same speaker as what's in the cab.
Most important, if your amp says it expects an 8 ohm speaker, then make darn sure your speaker (or your speaker cab) is 8 ohms. Match them.
