Going fixed cheaply?

[librarian]

This is what I did, no problems with not having a lock ring yet, but then again I don't do much leg braking.

If it's a cassette hub then you can fix it for free but it'll take a little work on your part.
Remove cassette, undo the three rivets/allen bolts at the bck to split it.  Sellect the sprocket that you want. Keep the spacers.
Now take the drive side cones off of the hub.
Take the axle out.
Whack a big allen key down there and take undo the freehub body.
Here's where it' going to be a real pain.
Look into the top of the free hub body and locate the little notch.
Bodge up a tool to fit it and dissasemble the freehub body.
Ping, ping, ping - that as the springs that hold the pawls up.  Retrieve them from under the fridge.
Degrease the whole thing.
Pack it out with a good glue such that the pawls are held ingaged.
Slide some HSS rod down the slot down the side to lock the freehub body.
Reassemble.
If you don't trust the bodge that you've just made then you might lso like to weld across the outer seal.
Put back on hub. 
Put axle/cones bck together.
Use the spacers from cassette and some old plastic plumbering pipe of the correct diameter to space the sprocket for your chain line.
Hey presto - done.

However, be warned, you might find that within the time that you've taken to do this you could have got a part time job and earned enough to pay for a real fixed hub.