I suggest that you should not buy links. Instead, do it with hard work. Having back links is not an overnight job. Buying links may give you immediate results, but in the end your site will suffer the consequences of various search engines most especially, Google. But it is still up to you. This is just my opinion. Good luck anyway.
If you buy links you must make sure that it is from a site that is relevant to your own otherwise Google will not see your link as valuable which means that your PR will not increase.