Nothing out yet kit wise as far as I know or even as a prebuilt diecast. The An-225 tends to be seen as a pre-painted desktop model available from importers of Russian stuff, but I don't know of a specific source for it. I think the scale might have been close to 1/200 or 1/288. The C-17 you would also be most likely to find currently as a 1/100 wooden desk top model. There might be a vaccuform kit out there of the C-17, but I don't think its in 1/200 scale (probably 1/144 if one exists).
Now on the horizon, Panda models of China has announced that they are doing a C-17 in 1/72 scale and Revell of Germany is also apparently doing an An-124 in 1/144 also (expect it no earlier until the fall of 04 I'd say). When the An-124 comes out, then I'd expect somebody will offer a conversion kit to turn it into an An-225 or somebody else might offer a dedicated kit soon after if the An-124 sells well.