There are various Dallas Maxim 1-Wire interface adapters and sensors available from Sheepwalk Electronics. Some kits are available unassembled.
Velleman do not supply any Pi-specific boards or accessories but they do sell generic USB devices. I have used the K8055 and K8064. Their website is at Velleman. Most boards can be bought either assembled or in kit form.
One of the two main-stream suppliers of Pis. RS also sell every conceivable component that you might need. Can be pricey but if you have an account, next-day postage is free.
The other of the two main-stream suppliers of Pis. Farnell also sell every conceivable component that you might need.
Supplier of well-made Pi cases and other accessories (GPIO, USB devices, SD cards, etc.).
Supplier of Pi interface boards. Not yet tried out.
Supplier of Pi intelligent power switching kits for your Pi.