Start here: check whether this is already funded
A testicular implant is frequently funded, because it most often follows the loss of a testicle to cancer, torsion or trauma, and most health systems treat it as reconstructive rather than cosmetic surgery. It is usually offered at the time of orchidectomy, the operation that removes the testicle.
If you declined it at that time and have changed your mind, or if no one offered it, it is worth asking again rather than assuming the opportunity has passed. Delayed implants are routinely performed and are often still funded, which makes this the first and most valuable check before pricing anything privately.
Men sometimes decline an implant at the time of orchidectomy simply because it was one more decision added to an already overwhelming cancer diagnosis, not because they had genuinely decided against it. If that describes your situation, raising it again at a follow-up oncology or urology appointment, even years later, is a perfectly normal thing to do, and clinicians who treat this condition regularly will not be surprised by the request.