I'm in the same boat but am having a hard time finding places that sell used tri bikes; any advice?

I actually disagree with the other posters. I think you should buy a road bike before buying a tri bike. Why?? A road bike is much more versatile than a tri bike. Generally, they are better for climing (and descending). They have a less aggressive geometry, making them more comfortable for long training rides, especially early in the season. Also, it's a better bike to ride on when on a group ride. You shouldn't use your aerobars when riding in a group or drafting off someone. I would recommend going to a reputable bike shop and telling them what you want to use the bike for and what your budget is. Someone at the shop should be a certified fit specialist. If not, find a shop that has one! The most important thing in buying a bike is fit. For example, people will say they want a Trek because that's what Lance Armstrong rides. But, if the Trek doesn't fit them well, then it's a piece of junk!

You can also add aerobars to a road bike and get a decent aero position on it. It won't be as aggressive as a tri bike, but for now it's a better idea. Then, if you stick with the sport and want to buy a higher-end, more competitive bike, spend the money and invest in a good tri bike.