Yup, a local motel, RV park, that sort of thing in town seems like your best bet, or at least has worked well for me other places; I've not started from Cascade Locks, or at least not and left a car there.

I have left a car at White Pass a couple of times; the folks that run the motel there were easy to work with, and they have a nice large parking lot so no problem for them during hiking season.