Hello And welcome,
The C6602A cartridges are actually plentiful on ebay and also fairly common in cartridge suppliers. The Q2299A (the carriage) is indeed a challenge to obtain. I still have a stock of them for sale, and there are some other suppliers throughout the world (except for ebay) that sell them. Depending on where you live, I might be able to help you find a supplier.
Making one yourself might be a bigger challenge than it is worth. Mechanically it is no problem, it is fairly straight forward to make. The problem is making electrical contact with the 13 contacts. The C6602 has the contacts on the bottom, You will need to create a system that reliably creates contact with all 13 contacts, all the time. This needs to be flat, otherwise the distance between the print object and the cartridge will be too big. It is not impossible, but not worth hacking yourself when the alternative costs €10.