Hi there. Always nice to see another maptool user find their way over here. I'd certainly be interested in seeing the sketchup battlemaps you've used if you're willing to put them up.
Maptool has a few tilesets that you can download directly from within the program (under File-Add to Resource Library... ) but I'm guessing you've found those already. As for the rest, well dunjinni is a good resource for objects to place. Once you're into the business of making maps from scratch however then you're into the long grass of Gimp/Photoshop and others. Once you've settled on a tool of choice, check out the tutorials forum. There's loads of tutes on there for battlemaps in different styles. However the best plan is probably to post up a sample map and we'll chip in with advice when you get stuck.
Welcome to the forum!