There a plenty ways of buying links, a general rule of thumb when buying links is the easier to obtain the easier they are for Google to detect.....
Think outside of the box and try and avoid pools of sidebar or footer links, which are probably the easiest to detect as paid, particularly when they are labelled 'Sponsored Links' or similar!
I honestly do not recommend that you try and buy links at all, these days Google try very hard to eliminate their benefit.
Pool your time into content generation instead and good luck.
If links are bought purely for traffic reasons - then they are just fine.
To the OP - look for blogs, forums, sites similar to your niche and with a good traffic stream and drop them an email. Ask them nicely if they would want to sell you a link on their site That always work.
There is another way around this. You could outsource your link building campaign. This is not classed as purchasing links and removes the hard work. Try sites like elance and scritplance but be sure to be very strict on the criteria you set for link building.