Where does everybody buy their inlays for their builds?

I'm looking for some 6mm Abalone dot inlays, but I need a 3mm thickness.
I'm not sure where to buy from in the U.K. One guy on eBay sells them fairly cheap, but they're too thin.

Anybody know the best place to get fairly cheap dots that size?
If nobody has anywhere they can get my sized inlays within a reasonable price in the U.K. then what's the best way to get the thinner ones to the right height? I thought if I added enough glue and very carefully slotted it in, then wait until the glue dries, I can then start to file down. But that will be very tricky and fiddly.
I need 3mm thickness as I'm replacing some old plastic inlays. The holes are just under 3mm deep.
In that case, get some plastic rod of the correct diameter, cut it to length, then use a laminate of plastic and abalone to inlay. That's the cheapest option, and I don't think you'll find 3mm thick cut shell.