I also have the sonoline b. I got this based on many recommendations on here. I got it on e-bay and brand new paid about $50. It's pretty basic, but does the job. I wasn't interested in spending more than $50, so that's what brought me to this one.
I didn't find what is for sure the babies heart beats until like a week ago. However, it looks like you're further along than me, so you should be able to find it more easily.
We LOVE having one. We have a high risk pregnancy, which as come with many complications. Being able to listen to their heart beats every day is very reassuring.
I just use 'My Babies Beat'..the iphone app! it was $3.99 and I can listen to her heartbeat whenever I want using regular headphones!