You can either buy one or boost an existing domain. Keep in mind that a PR value is for an individual web page, it's not the same for a whole site or domain. However, the PR will spread throughout a site via internal linking. If you're link building with the main goal to get higher PR, look for web pages with high PR that have a small number of outbound links and place your links there.
You should try to hire some SEO service. Or if you want to learn it yourself, try to submit your site to major search engine. You should do more linkbuilding.
there are two ways for you to get high pr domain. 1. Buy them 2. build quality links to them (maybe thousands links) I guarantee you will get what you want
You must hire someone to help you get that high PR. Get quality backlinks. But if you dont have enough time, you can buy some domains with high Pr but is little bit expensive.
If you mean how to build pagerank, this is the wrong forum. If you mean where to buy a high PR expired domain, that is a wrong tactic since it will lost PR overtime unless similar content to the original is held and updated.