Possibly buy a used engine at the monthly auctions -as you're in the US you could get KLR (www.klrautomotive.com.au - they provide a buying service) to bid on it for you and arrange transport (at a cost).
That would be the best solution.
4BD1 engines are available from a variety of sources, probably in the US, too. But by far the bulk of them will be for the Isuzu NPR range of trucks and won't have the Land Rover bellhousing adapter fitted. This part is fairly rare to come by separately, and if you want to make up a bottom end to swap into your vehicle you could re-use the adapter off the old engine, however if you using it to fit into a Land Rover (with an LT95 or LT85) and you don't have an engine already, an NPR (or industrial type) 4BD1 is not going to work without the adapter.
All of the engines sold through AFM so far have had the adapter fitted.