Where's the best place to get them when you break 2?
We have them, but let me give you a tip to SALVAGE the ones you 'broke' and SAVE the money! Normally, the plastic tabs break.
You could use some SILICONE SEALER, Black or clear,, and put a line of this goo around the hole in your wheel and a thin line on the cap. Press the broken cap into place. Clean off the ooze......
If the wheel is OFF....just put some small weight on the cap for a day.
If the wheels is ON, carefully apply some masking tape at opposite sides (ie 3pm and 9pm) for a day.
Now, the cap will stay in, and if you have to take it out for some reason, press from behind....and the sealer will peel off if/when needed.
[If you dont want to bother we have new in stock, under $70, prio mail USA $5, CAN $15 ]