You will find many sponsors here with any part you will ever need. Also the for sale section is excellent.
You will also find many tutorials and others questions on the modification and repair forums. When looking at a bike for sale.
Make sure it runs,
Make sure the suspension is solid,
Ask about engine work and who did it.
You can look at a few things to see how the upkeep was in general.
Was the gear oil changed regularly?
Are the sprockets and chain fair?
Are there any leaks?
Basically you just don't want to buy a bike that needs a lot of parts. But I'm sure after you start working on it.. you won't be able to stop!
Its fun and pricey!