Here are a couple of resources that will help you get in touch with your inner grunge designer.
Urban Dirty offers a selection of free background textures that range from grungy to gritty to slimy. The textures are asymmetrical so you cant create a seamless tiled background from any of them, but theyre rendered at 180 DPI in small, medium and large sizes.
Go Medias Arsenal site offers textures, vector art and fonts that would appeal to just about any skate rat or tattoo artist. The art is offered in reasonably priced royalty-free collections but you can download free samples here.