How far along are you? If you still can sleep on your stomach, I don't think you'll be causing any harm. Once you're bigger, you physically won't be able to!
There are tons of pillows out there, I got a theraline and it has helped. I bought it because it seems quite versatile and I might use it when breastfeeding once bubba is here.
I had a dream the other night that I'd had my baby (in the dream it was triplets!) and when I half woke up I tried to roll on my stomach and was really sad, thinking "no-one told me you wouldn't be able to sleep on your stomach once you'd had the baby!" - then of course, I realised it was just that my massive bump was still in residence!