I know this may sound silly, but MS Flight Sim is the best helpful yet cheap way to prepare for the instrument stuff. I dunno about ASA like someone mentioned above, but the regionals that I have interviewed with were looking more for instrument skills and scanning than how to fly a Dash 8, etc. (Because they know you've never flown one and that's what sim training is for... but they're not there to teach you how to fly instrument, that's stuff you should already know but some people are actually very shaky on it.)
What I did was read the gouges and go over generally what the sim ride for that specific regional would be like, then review all the different possible approaches and holds, etc. and shoot them on the Flight Sim using an aircraft that is most similar to the sim that will be used in the interview.