If you've replaced both the belts, and are using original equipment belts replaced as a set, then one of the other areas to look at would be the variable speed pulley itself. The center sheave should move up and down very freely, even if you try to cock it to one side by lifting up on one side while pushing harder down on the other, or vice-versa. There should be no lubrication on the shiny center part.
I'm assuming you have not made any adjustments on the linkage? And that you have inspected the springs to all be in place and not stretched? Those are some other areas to look at, but my experience is that 80% of the time it's the bearings. You can do a visual inspection for movement to verify before tearing into it.
As far as price and part numbers, it takes (2) of(NNN) NNN-NNNNand (1)(NNN) NNN-NNNN They run between $2-$3 each, depending on where you buy them. A good online source is M-and-D since they are reputable and sell at a discount. (No, I'm not affiliated with them).
Here's the procedure for doing the adjustment just to make sure:
When operating the unit initially or after replacing
the belts, there will be little difference
between the highest two speeds until after the
belts have gone through a break-in period and
have seated themselves into the pulleys.
1. First, adjust the speed control lever by pushing
the clutch/brake pedal forward until the stop on
the speed control rod is against the running
board rod (or until the stop on the brake rod is
against the frame, if so equipped). Have
another person hold the pedal in this position
as you make the following adjustment. Place
the speed control lever in parking brake position.
Remove the hair-pin cotter and flat
washer, and adjust the ferrule on the rod so it
is against the back end of the slot. Then
lengthen the rod one more turn. Replace the
flat washer and hairpin cotter. See Figure 7-25.
2. Adjust speed control link as follows:
a. Start the engine.
b. Place the shift lever in neutral position.
c. Place the speed control lever in high
d. Release the clutch/brake pedal completely,
then slowly depress the pedal all the way
(to park position). Hold the pedal in this
e. Turn the engine off.
f. After engine stops completely, release the
g. Position speed control lever as follows:
(1) 7-speed units—Place speed control
lever in second position.
(2) 6-speed units—Place speed control
lever between first and second position
(hold in this position).
(3) 5-speed units—Place speed control
lever in first position.
h. Remove the cotter pin and flat washer
which secures the speed control link to the
variable speed torque bracket assembly.
i. Push the clutch/brake pedal backward by
hand as far as it will go using light pressure.
Hold it in this position as you thread
the speed control link in or out of the ferrule
until it lines up with the pin on the variable
speed torque bracket assembly.
j. Secure speed control link to variable
speed torque bracket assembly with flat
washer and pin cotter.