You can generally find inexpensive monitors on craigslist, thinking around the 17" size for around $50 each. Larger will naturally start costing more. The nice thing about monitors is .. they are easy to reuse after the project display is over. So unless this is a permanent install, you can loan monitors to the project.
As for smaller monitors - I was thinking about something like these.
http://cgi.ebay.com/4-3-TFT-LCD-Car-Reverse-RearView-Color-Monitor-CZ7-/170537635936?pt=LH_DefaultDomain_0&hash=item27b4d5d060#ht_5577wt_1139
http://cgi.ebay.com/7-TFT-LCD-Color-Screen-Car-Monitor-rearview-camera-VCR-/230503864038?pt=LH_DefaultDomain_0&hash=item35ab19d6e6
or in the USA
http://cgi.ebay.com/7-TFT-LCD-Headrest-Monitor-/170590726834?pt=LH_DefaultDomain_0&hash=item27b7ffeab2#ht_2334wt_905
If you need multiple, you could most likely work out a deal. Also note when ordering from china .. be ready to wait a long time in most cases (i.e. two to three weeks can be normal).
There are also quite a few low cost DVD players out there in e-bay land and maybe your local pawn shops.
As for scavenging them, my opinion is you will spend more time trying to get videos playing on odd parts pulled from machines then you would want to spend.