The one from the 70's era cars is a hard plastic ratcheting armrest. It is kind of retro, but if you buy one, make sure the seller provides the triangular piece that mounts on the seat. You will not be happy without that one. Then, the fabric armrests. Those will bolt into the big bolts on the driveline tunnel at the rear of the center console. The catch there is you need teh center console and the underlying reinforced screw acceptors inside the console. Your 79 will have those. Some late fabric rests even have cupholder trays.
Finally, the deluxe armrest for the 90's 240s may work. Volvo went back to hard(er) vinyl and front and rear cupholder trays. These rests might still be available from the dealer. They mount on a big steel frame that fits inside your center console. I bet you car will take that one too, so any armrest should be fine.