It IS risky.
Many, many suppliers, honestly difficult to recommend one but I have not been disappointed with two bought from Wallenpower on amazon. They are INEXPENSIVE, no-frills shrink-wrap only, but IMO represent a reasonable value.
If you are willing to pay several times that price for higher quality, and deal with a reputable individual, there is Grin and Em3EV.
Double-check your voltage and controller capabilities, may need to replace controller. Space and shape requirements, Em3EV can custom make, they are a manufacturer. Owner known as Cellman, way back.
More AH equals more range. More volts equals more speed, BUT controller must be capable or new parts needed.
What to look for? Well, would you like to study individual cell models discharge curves and lifetimes and contrast long-life versus hi-power, then match that to your intended needs?
Cellman will let you specify not just what brand, but what model of cells you want. Not sure if current supply-chain issues allow this today, though. He in an Englishman living in China for decades, and a pioneer in the ebike industry.
There is another common vendor others will mention, so I will say this: Due to my personal knowledge and experience, under no circumstances would I purchase any product of any kind from Luna Cycles.
