The hard part is finding high PR do follow blogs in the first place.
If you have scrapebox, and can find competing sites that have these comments you can trawl for more and more targets, by extracting the links from the Hi PR blogs, then checking the PR of the sites they link to and look for commercial domains.
If you find commercial / affiliate looking domains that have PR, and have commented on those kind of pages, chances are they have dropped links on other comment pages too... so, again using scrapebox scrape up the lists of backlinks to those domains looking for Hi PR blogs.
Then extract the links from those... keep going for a couple of hours and you should build a decent target list.
Of course this relies on you having a "seed" site to begin with... either a Hi PR blog that has been used for links, or a target site that has been pumped up with these links...
There are tutorials on how to do this with scrapebox... but in essence you are using just two things:
yahoo backlink scrape
link extractor plugin
After that... follow the adice of others and dont use "names" like "cheap car insurance" and dont leave generic comments like "wow great post, just subscribed to yor blog"
Whilst sellers of blog comments may tell you their comments are so clever they will get approved on any site... anyone who runs a blog will tell you they can spot those shitty comments a mile off... probably after the second or third word... and anyone who has a Hi PR blog will have been around a while, so dont waste your time dropping shit comments on good blogs.
You need to READ THE POST and leave a RELEVANT comment to stand any chance of approval.
Better to get one approved every 30 minutes than 5 deleted.