These collars (offset bushings actually and sometimes called "slugs") are available through most parts stores (though many will likely have to order them).
MOOG and Specialty Products are only two of many suppliers. Your alignmnet shop should have these in stock - if not, I would strongly consider backing slowly towards the door....
Now - the tricky part... these slugs are available from zero degree to over 2.5 degree offsets (depending on brand) and can be installed in several positions... Knowing your start caster/camber reading and the existing slug offset are crucial to achieving a desireable caster/camber setting.
The "magic".... it isn't enough to get the alignment readings inside the "window" (manufacturers specs). The settings on one side influence the settings on the other...
A vehicle will pull to the side that has the most positive camber. A vehicle will pull to the side that has the least positive caster. And caster and camber can be used to offset each other. (Don't worry, this part can confuse even a lot of journeyman techs...). The idea is for the alignment tech to induce about a 1/2 to 3/4 degree pull (using cross-caster and cross-camber settings - this would be the side to side difference) to offset road crown.
As ever, it is vitally important to check toe in after any front suspension or steering linkage adjustments have been made.
