Petsmart carries a set that has a 1&1/4 comb attachment.
Andis 8 Piece Large Clipper Comb Set - Grooming - Dog - PetSmart
I have an Oster clipper. I got several longer combs at Sally's Beauty. You can buy them separately there. They are supposedly universal.
Oster - Universal Clipper Comb Attachments
You will also need a #30 or #40 blade to use under the comb. Most clippers just come with a #10.