Hi 5:12,
Every bike shop has popular batteries in stock, typically OEM quality (read: Yuasa).
These days there are alternatives, like Gel batteries.
Gels batteries don't leak or dry out, 100% maintenance-free, and will work fine installed sideways or even upside-down.
I've had very good luck with inexpensive (Chinese) GS Gel batteries, which cost only $59 - 69 bucks.
A third choice is high-tech 'racing' batteries which are physically very small and light weight (but not inexpensive).
OEM Yuasa batteries have the largest selection and you are pretty well guaranteed the exact fitment for your particular model, ranging from $69 to $169 bucks.
So:
If you have an unusual or obscure model, OEM might be the only way to go.
Check and see if there's a GS Gel in the book for your ride though - if there is, you're in luck.
Batteries are rated in Amp-hours ie. 10, 12, 14 etc.
Then apart from their capacity, different models will be the same battery however terminals will be on the top, or the side, or unique location specific to the bike model.
Some bikes take really unusual (therefore rare) batteries, you're never gonna find at CTC, y'know?
Then there are the SuperSports; for example R6 and R1.
They use super-premium Yuasa high-energy MF (maintenance-free) which are sealed like typical car battery.
Often base-model bikes can upgrade to a MF battery for a few more bucks...
But more often you'll find R6 and R1 owners in shock (HOW much you say that is!? Yikes) downgrading to the $69 with the six little filler screws on top, just like 1978, heheh.
As usual, quality costs more and (when many miles from home) a battery seems not a good place to cheap-out.
Whatever...
Batteries need to be load-tested; it's the only way to be sure they've "had it" and should be replaced.
Many new, expensive, even MF batteries fail load-test though, and their warranty is short.
This is why I'm into the Gels, they're great!
All bike shops have catalogues from Canadian distributors Motovan and Parts Canada, and GS Gel batteries are in there.
If you need to order, typically it takes ~72 hours or so (3 business days).
Regards