Before you go blow some serious coin, take the following into account:
1. 90% of your problem I bet is simply getting over your nerves in the interview. That will best be cured with some face to face interview prep. If you do it over the phone (and I did one as well as a sit down face to face) you will help your situation, but you really need to be looking at a real interviewers eyes when you respond. So even if you need to go to your neighbor or wife/husband for some practice face to face, do that and 1/2 the battle is won. Go out of your way wherever you are to be personable with people, so that you feel more comfortable yapping ad lib.
2. Ask your friends what they really like/dislike about you. Of my buds who didn't get hired here at SWA, one was a jabberbox and the other was stoic. I thought they were great guys but you only get one chance to make a first impression as they say. You have about 20 minutes to get the interviewer to like you, or at least not hate you. Ask your friends how you might screw that up and what they suggest to fix it.
3. I think who you go to doesn't matter as much as if you get to sit down face to face with them. SWA specific interview prep is nice, but who knows what questions you are really going to get that day. I'm sure a local firm in town does prep work (head hunters) for cheaper and in person compared to the ones most circulated on the aviation board. Once the stage fright is gone, just practice with your friends/spouse, tape and critique it.
Good luck