I have heard that the best place to buy it is from beamfarm.com You will have to wire it up yourself, it's not hard.
It should come in what ever lenght you need...they have it in roll form...probably like 50 feet.
Check out the temp contol devices while you are there...you get what you pay for so...there you go
A rheostat is a dimmer...you adjust the dimmer and measure the output of the heating device. Works well in a stable enviroment. You could make one with a light dimmer switch or you can buy them.
A thermostat will measure the temp and adjust it's output to maintain the temp within a set range. The cheaper ones turn on and off while the more expensive (better) ones will pulse the power to maintain a constant temp...
Good Luck...Someone will give some more info...I have to go home