take the bus ($1.00) as far east as it will go(Harrison Rd). continue walking about a mile down to Houghton Rd and head south. From here you'll probably want to try hitching out to the interstate, seeing as its quite a lengthy walk. it may also help to hitch out to Rita Ranch, a prominent housing development much closer to the freeway. There are plenty of good spots to stand just before the on-ramp.
Take the bus ($1.00) to downtown Tucson then, or if you're already near by, walk to speedway and head west to I-10. it should be rather easy to find. This ramp attracts most people from the east, west and central parts of town. Right now, however, there is a lot of construction going on and a number of ramps are closed til 2008 or 2009. All exiting and merging traffic is diverted to the frontage roads, so you might want to try your luck there.