As mentioned above, but not noted, it's vitamin D3 which animals get from UVB, also called calcitrol, which is responsible for proper management of calcium in the body. Snakes would get it from liver, as liver is really the be all and end all for nutrition when it comes to meats. Calcium is gotten from bones.
If you're that worried about UVB for your snake, take it outside when handling it. Even on overcast days, the UVB can be high, and not much is needed for synthesis of D3.
Disclaimer: not an expert on vitamins when it comes to snakes, but I have studied the hell out of vitamins in the past, and really vitamins are the same for all life higher than bacteria. If many of them have the same function in plants as humans, pretty sure they have the same function in snakes.