$15 each is a crazy price! They are available from the usual suppliers and from local engine rebuilders, if one is lucky. It did take quite a bit of searching to find some a few months ago. Technology has changed so much that newer engines do not use these plugs, and most auto supply stores no longer stock them. Once found, they sell for one dollar or less, each. The Healey block uses two sizes of freeze plugs.
After one freeze plug started leaking on the newly installed engine in the BN4, I replaced all of the plugs on the driver's side of the engine. After doing that, I noticed a very slow drip from the freeze plug on the rear side of the block. Fortunately, there was just enough space between the bottom of the curve in the firewall and the bell housing to change that freeze plug without pulling the engine - working from inside the passenger compartment. It took about an hour. Problem solved.