Which country are you resident in? TMC are the official UK Distributor (http://www.tmc-uk.com) and they might be worth dropping an e-mail to. There's also a company called ExpertsInStorage which supply Thecus units and spare parts (http://expertsinstorage.com/manufacturers/thecus.html)
Given the age of the N2100 I would be surprised if you can find a motherboard. It might be time to replace the N2100 with something newer.