There is still a lot of demand worldwide for the HP15c calculator. While the HP42 was arguably a better calculator, the sheer elegance of the HP15c beats it every time. All keys had three functions - at least, since some key labels could be understood differently if you were working with numbers or matrices (yCx and yPx), or as matrices or program labels (A to E keys).
Si ti is no surprise that people want to get hold of an HP15c "on the cheap". the easiest way is to reprogram a current ARM-based HP12c. There was a recent discussion in hpmuseum.org on hp15c firmware for the old Hp15c LE edition issued in 2011 - and it was posted there ( I will not do it here due to copyright issues - at the end this is a business and it is better to stay safe)
To do it you need a cable to connect your computer with a JTAG connector like the one in the back of the calculator. these connectors are made of POGO pins, and are increasinglt rare:
It is the 6-pin connector in the middle. This is a recent HP12c calculator. Beware with the cable: there are two types of HP12c ARM-based. The ones prior to 2015 were based on an Atmel AT91SAM7L128 and the communication protocol is serial RS232; while after that there was a new model based on another chipset, the Atmel AT91SAM4LC2CA, and here the connection is USB !! (with the same outward connection)
I enclose here a screenshot of hpmuseum.org discussion about the pinout of the two calculators:
You can get there more information on how to get the cable. Good luck!