Rubber window seals are a constant source of frustration for car restorers (and let's not forget the RX-7 Convertible is getting on for 40 years old now). The seals don't last forever (particularly in Australia with its UV radiation) and once the dealership supplies dry up, there's not always the demand for rubber manufacturers to continue to make them. Popular makes and models don't have quite the same problem, but even newer niche models like, say the latter-day Holden Monaro, is giving owners all sorts of problems sourcing replacement rubber parts. Social media might be your friend here. Try to find an online RX-7 owners' group and ask the question, Even if there are none readily available, some car clubs have enough members that specialist manufacturers will run off a batch of replacement parts, provided the club buys a job lot. If not, you might just find another RX-7 owner who has stumbled on to a replacement rubber that was originally made for a different car (or application altogether) that will fit and do the job. Don't rule out finding a similar rubber seal and modifying it to fit your car by slicing off part of it.