I did my first season of sprints on my mountain bike--just put slicks on and was good to go. Didn't know if I would like tris so went that route. If you decide to use your mtn bike you will see LOTS of people riding them.

If you really want a road/tri bike you could check craigslist for someone local and can try it out.

you can definitely use your mtn bike for your first race and there will plenty of others who will be there riding theirs. if you go w/ a new bike check your good local bike shop - a good bike shop will find a bike for you that fits you (very important) and fits your budget and not push you too to buy a $4k bike. i'd really suggest this over buying a used bike when you don't know if it the right size/fit for you. also starting a relationship w/ a local bike shop is really helpful if you need something fixed in a hurry down the road

You're right. I have a totally servicable tri bike that sits in a spare room. And for sentimental reasons it still has dried Gatorade on it from the last time I ever rode it. I just can't bring myself to dismantle it.

But to answer the post, you probably could find a place to rent. A large LBS will have bikes to rent. Maybe not TT/tri bikes, but at least road bikes. But if it is your first race, or first season, ride whatever you got until you know you really want to go 'whole hog' into tri.

Since you've got so much time before your race, start putting the word out that you'd like to borrow a road bike. You are average weight and height for a guy and soooooo many people purchased road bikes at some point because they "planned" on taking up cycling. Now the expensive bike is collecting dust in a garage or basement.

Ditto all above. Mountain bike is fine for a sprint. And, if you are new to road biking, you may find that a road bike serves you better than a tri. At least that was the advice I got when I asked a similar question before and I've been very happy with my road bike.

Good luck!