I got 40 ft from a place that test hoses for the Fire Department. Didn't cost me anything. Just had to cut sections out of it. Find out from your local fire department who tests there supply lines. Worked for me.
In utah you can get it from Evco House of hose. I'm sure most cities have a hose and rubber supplier. It doesn't specifically have to be "firehose" just enough psi rating to take the pressure that you want to press.