VonsMog.com has some inventory, worth checking it out but definitely hit or miss. Most of his stuff is 406/416 rather than 419 specific
Expedition Imports has about the largest inventory in the US, just because its not listed in the inventory its worth opening up a ticket and inquiring about a part (supply the NSN and the part number from the parts manual) as getting everything inventoried is a work in progress.
Lots of folks who attend military auctions buy old SEE inventory and list on Ebay. As I have commented in the past, some of the prices they ask appear to have been picked randomly (or they are looking for someone with deep pockets).
If the parts you need are Unimog Parts, (versus added on SEE parts), Couch Engineering (couchoffroad.com) can order any Unimog part still stocked by the factory. They import a container of parts from Europe on a routine basis. In order for them to help you, you need to get the VIN number. Its stamped on the frame just ahead of the passenger side wheel. It will start with WDB and then have series of numbers (note its a non compliant VIN ). A dealer has access to the Mercedes Electronic Parts Catalog (EPC) and they can enter the VIN and get a read out of the exact build sheet of the specific SEE. Just be aware that Mercedes parts are very pricey.
I have had luck just searching for the Mercedes Part Number on google and see where it leads me. Be aware that there are a lot of sites in Europe that may pop up that are just middlemen (same as searching NSNs) unless it goes to actual seller I ignore them.
Be aware Unimogs are far more common in Europe and the OM352 engine has been used everywhere, non OEM parts are lot more available and lot less expensive in Europe. With the Euro cheap against the US dollar, I have bought several items from Europe lately. If its lightweight and can be air freighted, its not that outrageous to ship but forget it if its heavy or bulky. Usually it comes quick but I just got a service CD for my 1300 that took a month to get delivered
Westfield 4x4 has a "store" on Ebay, they have been expanding their inventory of mostly running gear parts.
Atkinson Vos
www.unimog.co.uk has a lot of unimog parts. They are in the UK so factor in shipping.