This is definitely a good one-stop shop for gear, informaiton and discussion. You can purchase most of what you need for beginner lines, all the way up to more advanced ones. The forums in the "Community" section are a great resource as well, and an answer to any question can be found in them.
Here you can shop for gear and read some informative tech articles.
This is Scott Balcom's site and it offers some good insights into the art of Slacklining. It also has a few items for sale.
There is a wealth of information spread out on blogs and countless little sites. Google should take you to some answers with the right keywords.