The easiest hollowing tools to make for yourself are the Orland Tools (Google for many examples) - or here (
http://fromthetree.co.uk/hollowing-tools.html) - George is a member of this forum
If you can get hold of David Ellsworth's Book - Ellsworth on woodturning, he uses a lot of tools made himself from old screwdrivers and the like.
I've made both of the above and they work well.
Next up there are the bought tools, the popular tool at the moment is the Munro. It is very good but best on end-grain and fairly unseasoned wood. Take a look at the Toolpost (
http://www.toolpost.co.uk/index.html), they stock many of the various tools and as a corporate memner of the AWGB will offer you a discount if you are a member yourself.
For more companies that offer discounts check out the AWBG Page Links tab.