8 1/4 gears?

So I have an 8 1/4 sitting in my garage with 3:55 suregrip...I am starting to think about going up to a 4:11 possibly 3:91 but have had a little trouble locating the gears....does anyone know of a good place to get these?..and before you ask the car hasnt been on a highway in 2 years and its a 360 with 600 cfm carb, mild cam, Rpm intake, headers, 3200 stall, and im running a 727.