The S&W 78G has not been made in a long time. It was made between 1971-80. I suggest you check the web under "Airgun Repair" and contact the shops you find, ( I was able to locate a set of grips for an old Crosman 38 this way.) But it may take a few calls to locate a set.
Take it to a gunsmith
Many different replacement grips are available for most Smith & Wesson handguns. Take the pistol to a sporting goods shop (be careful of local laws regarding transporting handguns) to find something which matches.
300-700 USD
GunBroker.com
Give you $20 for them. Thanks, Atson
Wood
crimson trace grips for smith and wesson 39
Best left to a gunmsith.
Ask a gunsmith for assistance.
chec at: midwayusa.com
Loosen the grip screws.
Need a sn to date it.