Our production was designed with very cartoon-y sets and costumes, so we built our own handsets from sheets of foam.  As I remember, we used 1-1/2" thick foam, doubled it so the phone really read from the house. They were about a foot long, maybe closer to 9 inches.  We glued the halves together (always an adventure) and base- and top-coated them.  The top-coat was one of the following: red, yellow, green, pink or blue and was also used on the five revolving upstage 12 foot by 5 foot rear wall flats.  The kids were on a number of platforms and the handsets looked great.  No noise if dropped and practically indestructable (not that you can easily destroy one of those old Bell-Tell Bakalite phones). 

I realize this doesn't answer the question of where to find real ones, but it's always nice to have a plan "B".  

This is the perfect subject matter for an amusing news release in your local paper!  Hit two birds with one stone -- publicize your show and ask the public to help you find the telephones you need.  You might even find some new volunteers for your theatre!  Good luck!

I love plan B! When in doubt use foam!!

I think I have used that one on more than one director!

You might find that when you need an older prop, the parents of the teens that you have cast would probably have an 70's or 80's style phone reciever at home.