The sticky on the electric car forum gives you info about the best place to buy the desiccant filter. That dealer also has the best prices on any Smart car parts that happen to crossover to some Mercedes model. The downside is that there aren't many.
Any Mercedes dealer that sells or used to sell Smart cars should be able to get you any parts you need. If you're lucky, the dealer may discount the parts from MSRP is you are a veteran, a member of MBCA , etc. Even with the discounts, Mercedes is still pretty proud of their parts and their prices reflect that. Many of the routine service items like filters, etc. can be had at the local McParts stores. A search of the forum will also give you online sources that others have used.
2014 EV Coupe 16,000 miles
2014 EV Cabriolet 6,500 miles