Rick, most modelling shops sell polystyrene cement. The stuff in tubes is a bit icky and doesn't work great, but you can get a brush-on liquid variety too, which sounds like what you're after. Whether it works on the particular kinds of plastic you've got, I don't know.

If you're using epoxy, make sure the surfaces are good and rough - a good rub over both surfaces with a bit of coarse sandpaper is essential - and then clamp them together in a vice.

Graham.