It basically depends how long/tight you want to rig your lines, and how future-proof you want them to be. You could buy some 25mm nylon tubular webbing and use the ratchet you already have to tension it. This would be ok, but you have to be careful when using 25mm webbing in a 50mm ratchet, as it can get mangled in the edge of the spool. It is also hard to get lines longer than 20m tight enough, as the spool tends to fill before the line is tight enough.
I would suggest buying some webbing and using krabs to tension it with a 'primitive' system as shown in this video:
http://www.youtube.com/watch?v=MdDzz5H2VQAThe most important thing to remember is that when the webbing goes through the krab on the line-locker for the second time, it must go UNDER the first strand. This creates a friction lock which locks the whole pulley system and holds the tension.
Adding a multiplier to this will increase the mechanical advantage by 3 (in theory), allowing you to set up longer lines by yourself (up to 30m).
http://www.youtube.com/watch?v=-gKMeeAk460This video illustrates the principle, but you do not 'need' the extra rope and pulleys. You can simply take the pulling end of the webbing (where it comes out of the friction lock), and add a linelocker and krab close to the friction lock. Then pass the webbing through another krab on the anchor, and back through the krab on the linelocker you have just added. Then pull on the webbing as normal. You will have to reset the multiplier every so often as it reaches the anchor.
You could also attach your ratchet to the anchor and use it to pull on the webbing (instead of or as well as the multiplier). This means you can keep pulling the webbing when it gets too hard, and you can reset the spool when it fills up (as it is locked by the friction lock).
Using this system is efficient and allows you to get a feel for tensioning with pulleys. With long lines (over 30m) pulleys are pretty much essential. I have just upgraded to a pulley setup like this:
http://www.youtube.com/watch?v=gL0UM0YWyUAThe principle is the same, it is just more efficient, allowing looooong lines to be rigged!
So... what should you buy?
25mm tubular webbing from a climbing shop. This can be used for the mainline and anchors. As you will be using it in the tensioning system, you will need more than the length of the line (obviously) so I would suggest 30m at least. If you ever upgrade to a pulley system then you will be able to rig the whole length of the line. Basically you can never have too much webbing!
http://www.needlesports.com/Catalogue/R ... -TPE-TM26Thttp://www.rockrun.com/products/Beal-26 ... -Tape.htmlIan has some Edelrid x-tube webbing which is quite nice too.
In terms of krabs, climbing ones should be ok for now (BUT DO NOT USE THEM FOR CLIMBING AFTERWARDS!), but if you are going to invest then steel krabs/maillions/shackles are the way forward.
I would also suggest that you have a look through the forums on here:
http://forum.slackline.com/index.phpThey are a mine of useful information from the best slackliners in the world!
Sorry for the information overload!
Hope this helps