Band shirts etc are available through a number of manufacturers:

Olympus Designs have a few different Murderdolls shirts availabe
Blue Grape also have merchandise

I know there's many more than this, but, well, it's 4am, I haven't slept for 2 days and I'm getting a little sleepy now.

Attitude Clothing Co Have a good range of red and black clothing at the moment, perfect for that Murderdolls look
Ebay.com Murderdolls things always being auctioned on Ebay
Ebay.co.uk The UK Ebay site
Roadrunner Records sometimes have rare items to sell or give away in competitions. Sign up for their newsletter to get information.

The Thirteen Dead Kids is Wednesday 13's forthcoming merchandise range. Keep checking the site for more information.