One thing you have to keep in mind is that when google is updating the toolbar PR, the results can fluctuate...
I built my own so don't know which ones are "reliable".. Did you try these two?
http://www.seochat.com/seo-tools/pagerank-lookup/
http://www.seochat.com/seo-tools/future-pagerank/
Oh.. and I found this awesome one the other day when I was revamping my tool.. even better for checking if the data centers are in flux.. However, I found it does have an "problem" as well. If one of the data centers is redirecting to a different one, it will still report the PR, but from the other data center. Not that big a deal. I'm also not sure if it actually reports the PR from the "default" toolbar host data center.
http://www.digpagerank.com/