welcome to the forum David,
you can get most parts needed from ebay but takes time to source them all,
gearboxes are approx £50 to £70, uprights / brakes etc tend to sell as a job lot, you can of course buy brand new front uprights from Rally Design, i think they are £99 a pair.
good luck with your build, any questions then have a search on here or ask away, its a very friendly forum & everyone is willing to help .
cheers
andy