Ebay, autojumbles, dismantlers & dealers. Just make sure you get a complete take-off kit of steering-box, pump, pump bracket, bottom pully, resevoir & pipework, try not to buy the parts separately as you will probably end up paying more (pump-bracket & pully are sometimes missing). You will also need PAS fluid & a new belt. Should be able to get a complete, good working kit for under £200.
Easy enough to do. the worst part is usually just getting the crankshaft bolt out to replace the bottom pully if you don't have a crankshaft locking tool (I have so an easy job for me). Single person working on the driveway should see it all complete in about 4-hours as long as you have all the parts to hand. A manual will show you how it all goes together and how to bleed the system.
