Where have you tried? If it is simply looking through websites and catalogs, then it isn't surprising that you haven't found one yet, as older stuff is often not listed in there, even though it is still available. Try going to a Honda motorcycle dealer, or any decent bike shop, and they should be able to look it up and order it. You could also contact
Royal Distributing they don't list it for the 86 in their catalog, but they are a Kimpex distributor, and Kimpex lists the Wiseco piston kits for your bike, in stock and overbore sizes. You will need to know what size your cylinder is at in order to know which piston to order. The cylinder may need to be bored to true it up. The best way to do this is have it measured, if it needs a bore job, then order the next oversize above the current bore, and have the shop set the cylinder up for the piston. A good bike shop can help immensely in this regard.
Your old piston is done, throw it on the shelf and forget about it, you can't reuse it unless it is still within tolerances and undamaged. I doubt this is the case if you're asking where to find a new one. You also need to determine why the piston seized in the first place. If it is an air leak, excessive wear, excessive piston to wall clearance, dirt ingestion or some other mechanical issue, that needs to be fixed before you rebuild the top end, or the new piston will not live long. You also need to check your bottom end to ensure it is clean, and in good shape. A new top end won't do any good, if the crank is ready to fly apart.