These are not too hard to find at thrift stores. The best place to look is in the black t-shirt section of the store. They usually cost between a dollar or two. I have even purchased a few for .49 cents on half price days.
It amazes me the stuff that I find at thrift stores. I have found Harley shirts in my New York City thrift store representing Harley dealerships from all over the country from Montana to Arkansas. I guess while Harley owners are hitting the open road, along the way they stop at every Harley Dealership to check out the apparel.
I have included screenshots of two of my recent sales. Not too bad for a t-shirt purchased for less than $2.00. If you are avid thrift er you might want to hold on to the Harley tees and list them in a lot when you get a good group of shirts. I have seen lots of 8 used Harley shirts going for $150.00 and up.