Avoid paid link services. You can be penalized by search engines. If you decide to go on, find reputable sites relevant to your business with high Page Rank.
I figure out that you may want to read this post! It will help you on finding paid links that has a high pr and is relevant to your niche. http://www.wahb.biz/2008/08/get-a-good-back-link-for-a-niche-site/
Lyn is probably right if the link exchange service becomes famous then google will try to find ways to devalue the links you gain from it ... or penalize you
Just go ahead and contact webmasters like me. Pay us half the amount and the other half at the end of the month or middle of the month. That's it. You got the paid link. Only thing to make sure is don't get the paid link under any category: sponsored links or advertisements, etc. That will easily get both sides penalized by google.
if yuo are looking for paid links, then best way to do it is to find them yourself and pay for them. that way you know which sites your links are gonn abe on. you can find plenty of paid links on BST forum