I don't think you can actually get FREE plans.
Maybe, search the boat forums for pt boats seems like somone posted a good site.
Otherwise you can order plans from these sites.

http://www.ptboats.org/ for ordering plans, or http://floatingdrydock.com/

Make sure you order plans and "hull lines"

Building from scratch is somewhat cheaper. I have a lot of supplies sitting around, but I usually budget $150 for hull, and maybe supersturcture, for my ships.