Remington was supposed to be importing them from Baikal but never did. They even have a page on their site about them http://www.remington.com/products/firearms/centerfire_rifles/SPR_models/SPR22.asp
I had thought 30-06 but 30-30 would be fun or even 45-70.
It seems to me that the double guns extraction systems best lend themselves to rimmed cartridges. I'm sure they can be made to work just fine with rimless rounds, but the neurotic in me would simply be happier with a 30-30, 303, 30-40, and yes, the 45-70. I say this as one who all but worships the 30-06 and the .308.