Unfortunately this is going to be a tough one. I've spent a few hundred dollars over the past year hunting for blog commenting services that were worth anything - the results are very far and few between. Most of the commenting services that claim to do manual postings are using xrummer and other systems to automate the tasks - which of course means the comments that are made don't contribute to the blog and usually get marked and deleted as spam by the blog owners.
I have a gal that knows the fitness industry pretty well and we contract with her to write articles & commenting at various places - but she's a nutritionist by day and writes a few postings by night to make some extra money.
I've not found any reliable companies for commenting
There's the "pay per comment" type services out there too, but you'll spend so much time reviewing what it was they wrote, and then getting frustrated with the lack of command of either the topic or the English language, that you'll cancel the services pretty quickly.
If you find something, I'd definitely be interested in spending money with them too - but at least the couple of dozen places that I've used, the results are low quality and don't stay on pages too long.
What I have found however is the buysellads place to promote via other persons blogs - if you get your banner on the right site, they've been very successful sources of driving extra traffic.