If good value = low cost in your mind, here are some suggestions.
Mod a stereo cartridge - with a fine copper wire (maybe 24 ga or thinner, or even a single wire cut from a stranded bundle in a speaker wire) strap the hot pins (L & R) on the cartridge. Do this before reconnecting the headshell wire clips. Be certain the wire is taught between the pins and wrapped a couple of times around each pin. Trim away any excess.
Y adaptors - with a male to male Y connected to a female to male Y, insert that between your tonearm and preamp/phono stage.
Buy a new mono cartridge - Shure, Stanton, Grado, and AT all offer reasonably priced models, <$200. There is some question if all of these are "true mono" or simply strapped internally as in the mod suggested above. A true mono is the Denon 102 mono. It has only L & R connector pins. They are extra long to allow connecting all four headshell wires so you will have signal from both speakers.
Mod a stereo cartridge - with a fine copper wire (maybe 24 ga or thinner, or even a single wire cut from a stranded bundle in a speaker wire) strap the hot pins (L & R) on the cartridge. Do this before reconnecting the headshell wire clips. Be certain the wire is taught between the pins and wrapped a couple of times around each pin. Trim away any excess.
Y adaptors - with a male to male Y connected to a female to male Y, insert that between your tonearm and preamp/phono stage.
Buy a new mono cartridge - Shure, Stanton, Grado, and AT all offer reasonably priced models, <$200. There is some question if all of these are "true mono" or simply strapped internally as in the mod suggested above. A true mono is the Denon 102 mono. It has only L & R connector pins. They are extra long to allow connecting all four headshell wires so you will have signal from both speakers.