Hello also George,
As Alan said the backs from the S, S2 & S2a are interchangeable, they are not interchangeable with the EC models.
Any lens made for the classic Bronica will work with your S2a. Classic Bronica's are the D, Z, S, C, S2, S2a, EC & EC-TL.
Accessories, prism finders and such, are NOT interchangeable between the 'S' series and the 'EC' series.
There are more Bronica items available on E-bay-US. I have bought and sold internationally on E-bay-US with little or no problems.
Some sellers have had trouble, if you see something you like you can always contact the seller and see if they will sell internationally.
The major problem with the international sales is the shipping cost, something small, like a filter is not too bad. When the package weighed over 4 pounds the cost was typically over $50.00 US. So that is something to think about.
Most of all, enjoy your Bronica. At one time I had each of the models we have discussed, now I have limited myself to just the EC series. They're great cameras.