e-bay has been the cheapest place I have found latex tubing. I have purchased there several times to make hi-starts.
Light 3M planes, say under 65 ounces, can be launched with 5/16 latex tubing. If you plan to get into larger or heavier planes, you might wish to go to 3/8 tubing. If I was building a 3M hi-start I would use 3/8 tubing. That is 3/8 OD, outside diamater. This assumes a 1/16 tube wall but the wall can vary from 1/32 to 3/32. Naturally the thicker the wall, the stronger the tubing. So a 5/16 OD with a 3/32 wall might be comparable to a 3/8 OD with a 1/16 wall.
If you are trying to go cheap and buy from e-bay you MUST confirm that the tubing is a solid pice and not a box of short pieces. Both kinds can be listed and sometimes they don't say, you YOU must confirm this.
Here are some examples:
http://cgi.ebay.com/50ft-BLACK-RUBBE...item3f06aeb46a
http://cgi.ebay.com/UP-START-LATEX-R...item2a0e718f2a
http://cgi.ebay.com/UP-START-LATEX-R...item2a0e719ca6
You can join pieces to make longer rubber. So, for example you could join two 25 foot lengths to make a 50 foot piece. The method is discussed in the thread.
http://cgi.ebay.com/50ft-BLACK-RUBBE...item3f06aeb46a