Lett went out of business. Ruger used to have some of the grip inserts, but they don't show them anymore.
There was/is a guy at ruger forum making inserts that were said to be very good quality, you might check there.
.....tom
Will have to give John's(41Magnum),work a big plus 1. He did a matching pair of inserts for my GP/SP in Lace Sycamore. I really like the contrast with the stainless.
For years LETT GRIPS was the sole supplier for Ruger's grips.
When Ruger went to rubber type grips there was no more use for them.
So they closed down.