i believe that they sell casters as guitarcenter and most music stores. if you are at all nervous id go there and see what they have. then more importantly talk to someone to see if what they have would fit your amp or if maybe something like what you were looking at would work. best of luck.
Those casters aren't the best. I made an amp cart with them and they have alot of rolling resistance on softer surfaces.
I'm putting my GAS on hold
for a couple months in order to pimp my ride.

Don't judge me.
Those will work and yea its as simple as it seems to add casters. But you may want to go with 2 swivel and 2 fixed casters. With 4 swivels the cab way get a mind of its own at times.