Does anyone know of a cheap place to get a set of areo bars for my mountain bike, doing my 1st triathlon soon?

Not to put you off, as the tone of the other posters is a bit negative, but I would investigate replacing the fat rims/tires with thinner versions (if you have not done already) and perhaps removing any extra weight (mudgards, panniers, lighter saddle, road bike peddles) etc.
The proflle of a Mountain Bike is not really suited for aero bars anyway and the riding position compared to a road bike is more upright. Of course, once you spent the time and money modifying a mountain bike to be more like a road bike, you'll probably ask yourself whether getting a road bike was quicker anyway ;).
Perhaps look to see if you can hire a road bike for a couple of weeks - most Triathlon related shops do hires for not much per week.
As its your first Tri, you'll probably spend more time getting used to doing the swim, bike run thing all at once anyway to be too bothered about what type of bike you are on - don't underestimate the ammount of time you can save in transition as well - if you get you transition routines (swim to bike - bike to run) as quick as possible, you will save loads of time, spurring you on to make the individual parts of the tri better.

Performance Bicycle. I bought mine for $35, normally $70.

You don't need aero bars.

They might look silly on a Mt. Bike. Not that I am overly concerned about looks- but the purpose is to be sleek - and Mt. Bikes are not sleek. I've done about 20 triathlons without aerobars. I haven't won any - but you are not going to win with a Mt. Bike either, so good luck.

But you can buy aero bars at any bike shop - or on the web, someplace like performance bicycle or nashbar.com.

there are a lot of areo bars on ebay, however i agree with the other responder. You are not going to get a lot of improvement on a moutain bike-the areodynamicsare not goingto improve that much considering the tire surface you have on the road. Good luck on your triathalon; the swimming part is holding me back from trying one.