If you can get the part today from Radio Shack it's usually worth it. If you need 10 parts, or it's not in stock at Radio Shack, then use Great Plains Electronics because he specializes in pinball parts and that's worth supporting. Great Plains will also have info sometimes about appropriate part substitutes in pinball applications.

After that comes Digikey and Mouser.

That part should be fine. Here's the Great Plains equivalent for $.60 or $5.40 for ten.

http://www.greatplainselectronics.com/search.asp?pg=1&stext=tip42&sprice=&stype=&scat=

_{Post edited by rancegt : I forgot the link.}