Gotta love the 1 word answers!
Anyways, there are many options. Stock length is about 13.5 inches.
most any shock will work as long as it is eye to clevis. Look at the top of the shock, that is the eye, a bot goes through it.
the bottom is forked, that is the clevis.
Now, you can find some emgo/other no name shock that is black and looks like it has dual rate springs. From my understanding, they work ok but can be quite stiff. they are cheap though, about $80 or so, depending on the site you get them from.
If you want a better shock, hagon shocks are your best bet. They can be had for about $200. Dave quinn
http://www.davequinnmotorcycles.com/cgi-bin/webc.cgi/Hagon.html is a good place to go, you get to pick out the spring rate you want. the shocks are also able to be rebuilt.
Progressive is your next choice
http://www.progressivesuspension.com/These will run about $300, probably a bit better than hagons,probably not by a whole lot. You can also choose spring rates and rebuild them.
They do make good springs for the forks though and are recommended to replace the old things in the forks now.
You can go more expensive, works suspension and probably ohlins but unless you are getting into vintage racing, the cost is probably not worth it. Most are happy with Hagon's and progressive.
The ride IS the adventure. The destination is just to get gas!