The Challenger T&G brush is listed on the "Price List" page of the Challenger website, referred to there as "Grout Brush". Click on it to read about it & how to use it. Then call the Challenger toll-free number - 877-446-5887 - anyday between 9 and 5, CST. Or e-mail them at email@example.com.
The Home Depot TILE LAB line of stone & grout cleaners and sealants seems to work just fine, or you can go to any specialty marble & granite retailer in your area. They always carry one or more lines of these products as well, although probably at a greater mark-up.