I bought mine on aliexpress, but there's listings up on ebay too.
These are much improved over stock in the Burst Pro, which is cool since it's a great mouse with a dog's optical switches that even with a split button design manage to feel exactly as stiff and shitty as an old model Zowie FK1. The new ones are slightly lighter and quieter than Razer V2 switches.
Here's a recording of stock Burst switches vs the updated ones and Razer V2s so you know this isn't pure cope: https://vocaroo.com/16y7CPSJAtwm
If you buy some just get at least 4 for binning purposes, and adjust the 2 screws up front before putting the feet back on since fully tightened gives n