Not sure if you want a motorcycle design or the various related types of designs for bikers but Urban Threads has a bike design, tribal and tattoo type designs. http://www.urbanthreads.com/
Search for "biker", "tribal", "tattoo - there are more of this type of design on the site but this will get you started
Here is another site. This is the old AnnTheGran archive site on the wayback machine
http://web.archive.org/web/200303...
There are several motorcycles there all free
Ann the Gran has some nice ones.
And there is a free motor bike on Embroidery Designs 5th line down on the page. Love Chris
Designs by Sick have a few sets relating to bikers