Well I couldn't tell you where to buy one, but here's a suggestion!
Make your own, it's really not that hard to do. I usually make a half dozen or so at a time. I use 1/2" MDF or oak.
Take your factory insert and trace around it on your material, take it to the bandsaw and then carefully sand it to final specs.
If I use mdf I have to use tape shims, otherwise I plane my oak down to just the right thickness.
The other benefit to making my own is that now I have dado
inserts for eack common width I use, 3/4", 5/8" 1/2" etc.. basically a set of zero clearence inserts!