Jeremy,
For my oversized SGC holders (T3 sized), I am using sleeves made for Golden Age comic books made by BCW. They are 8" x 10 1/2" with a 1 1/2" resealable flap. I picked them up at a local comic book shop, but if you don't have one of those near you, I'm sure that you could order them online.
For the T201/T202 sized SGC slabs (or "tall boys"), I ended up finding some sleeves that work well from the website clearbags.com, as you can see from the picture below. Hope this helps!
