Are you using the stock spacers already? The speakers in the front doors should have stamped steel spacers that angle the speakers up and back and allow pretty deep speakers. You might have pulled the spacers with the stock speakers and that's why the newer ones are "too deep". Check to see if the stockers have the spacers still screwed on to them. Unscrew those, put them back on the doors, then attach your speakers to those.
If you have 6x8 Rockfords and they are still too deep with the stock spacers and you still want to use those, why not get some nylon or MDF spacers to move the speakers out more? 1/4" - 1/2" spacers should be all you need to get the right depth. You're only limited by the front of the speaker making contact with the door panel. You'll probably need to get longer screws for them to attach securely.
As for other drop-in 6x8's that fit, yeah, the walmart ones are nice. They have rubber surrounds, and aren't bad for $50 bucks. Pioneer also makes some slightly lower wattage 2-way 6x8's with cloth surrounds, which are better for running off head units or low wattage amps, but actually cost the same as the 3-ways since walmart has such a low price on them.
