Check your local yellow pages and find a spy shop, they usually have nice small cameras for sale. You could also get a networked camera (Toshiba and D-link come to mind) and hook them up through WiFi or wired ethernet. BUT; be sure you know the legal aspects of these things, not everything in your own home may be ok to do when it involves others.

Our local electronics superstore sells a 4 pack of tiny (2cm*2cm) cameras with transmitters for only $200, i'm sure you could find something like that on Ebay...