how big is the tree? you MAY can do it with a sapling, but once they get a few inches it is hard to do. i know folks who have used a section of PVC pipe around a tree to both protect the trunk and eventually straighten it, but again only on smaller trees up to maybe 2-3" diameter.

I googled this question and it looks like Gemplers had such a product but at this time, their site was down. I did not call them but they would proably answer the question if called.

Go to Awesometools.com. They have the tree trunk straightner.
Also treetrainer.com.