If you have some time to spend, you can do a search and make a list of blogs which have removed the no-follow tag for links in comments. Then comment on some posts there, making sense and staying on topic.
However,
PR is very important only in case you want to sell links on your site. Otherwise, a very good on page optimization (with the help of a few external links which are not very hard to get) should bring you amongst the first search engines results pages.