My mother bought me the Sonoline B Doppler off eBay for only $40. They work great and are amazing for your sanity as well!
I agree with Maw, that's the one I have and I purchased it from Amazon.
I've seen some people post on here about getting a Sonoline B doppler from ebay. They said it works good. I bought the doppler from babies r us and returned it because it was garbage and I couldn't hear anything and I was around 30 weeks. Try to find one that's 3 mhz or better if possible. The higher frequency the better!
I have one which I bought when I was early into the pregnancy, it's really good but honestly you probably will only use it a few times. I literally used mine about 5-6 times and I'm 37 weeks now. It's in the closet right now with 3 big bottles of gel.