I have a D 300 Gen 2 and instead of the normal thread protector ring that's on the front lens, it has an adapter thread protector that's can accommodate a mil spec 3x lens.  It works fine but    with the thread protector installed, I can't use the front lens cap; unless I remove the adapter and have the threads exposed.

Where can I buy this part and parts and accessories for the D300 in general?

Thanks.

could be easiest to measure the diameter of the front with the 3x adaptor on and then find a common lens cap to fit like the pvs14 cap or whatever is the right size

I've thought about that.