how would you go about scratch-building something that's basically a sphere? i'm trying to make something that's mostly round with a few protrusions and curves. what i did was model it in a solid modelling package then print out cross sections, cut them out of balsa sheet and putty over the staircase look but it's still a bit gritty. any better ideas?

You didn't say how large the finished article had to be. For small items, Plastruct includes hemispheres in their line of architectural shapes. As mentioned, craft stores have a variety of plastic items like beads and such, as well as styrofoam spheres which can be covered in epoxy putty for rigidity (avoid using solvent type putties or glues on expanded styrene foam, as it will melt into styrene goo). Can be glued with white glue or epoxy. Sometimes turned wooden spheres can also be found in craft or woodworking stores. And the toy sections of dollar stores can be a treasure trove of unexpectedly useful plastic bric a brac (the big chain stores, alas, are increasingly devoted to viideo games and RC toys).

i'm talking about spherical shapes about 2-4" in diameter. it's easy enough to find something round but i was hoping for something smooth and cuttable as well, maybe a sphere made of styrene sheet. that way i'll have the texture i need (or rather lack of texture) and be able to cut sections in the shape that i need.

Try getting pastry cutters and using Milliput or something.

May be a bit big but how about a ping pong ball or something similar

redleg