Good old ebay.

any system involving two separate bars, rails or what ever needs to be perfectly paralel. Even dovetails with adjustable gibbs only average out the highs and lows when worn. I think the THK's are good at absorbing small amounts of misalignment but you do need to get them fairly level and paralel in the first place.

the 5 bears site has a good 'how to' on setting up rails, using one as a master and dti gauges to set the other.

Are you doing a build log as well?