I don't know any specific vendors that focus on replacement motherboards for notebooks. I would recommend asking Toshiba the part number of the motherboard for the specific model you have. Pop that number into a search engine, or get the part from Toshiba or a Toshiba certified dealer (ask Toshiba for the closest dealer located near you). It is still best to get a dealer to do the replacement.
If you want to do it yourself, you need to realize that the warranty (if it is still active) is most likely going to be voided once you do the replacement. As for taking apart the notebook, it is not too hard, but you will need to be more cautious than what you may normally be accustomed, in regards to desktops. First and foremost, a good rule of thumb is to unscrew everything that you can before you go about your merry way. Since it is a laptop, your best bet is to start on the bottom and work from there. Once you unscrew the bottom, check and see if the keyboard can be detached. After that you will need to disconnect all wires underneath the keyboard (make a note of them so you can put it back together). Then, resume taking it apart from the bottom.
If you are looking for a general sense of a notebook's layout, I would suggest you read the most recent mobile review from us.
Note: It is possible (since the system is about 2 years old) that the replacement part is going to come clost to costing what an entire unit might...