There are several different slacklines on the market. The slackline that I use was bought from a company called Webster-Maroon
. I have a kit that they make called The Highway. It's a 50 mm wide line which is easier to use than narrower lines, although I would start on a 1 inch line if I was to do it all again. I'd rather take a little longer to learn on a narrow line in the beginning rather than relearn how to slackline on a narrow line later on. On the plus side, I really like the lizards printed all over the Webster-Maroon line.
is another big slackline company out there that sells all sorts of kits.
Technically, you can make your slackline. Doing it yourself may or may not be cheaper than buying a prepackaged slackline kit and may not be as safe. If you're thinking DIY, here are two other Instructables on putting together your own line: