You can write it yourself, then have a lawyer proofread it for accuracy / coverage.

There are places on the web where you can buy boilerplate contracts - but, just like writing your own, be careful that you fully understand what your own particular TOS needs to cover, and that it's in there.

A few years ago I wrote an article for PingZine about this exact issue. I'll go see if I can dig up my original Word file...