*1988 chevy blazer needs retro fit shoulder belts in back seat from lap belts. on 1990 Chevrolet Blazer

I have a 1988 chevy blazer that has lap belts in the rear seat, in order for my toddlers in booster seats to ride in the back I need to convert the belts to shoulder belts, where can i get the parts and info I need to do this? I have to make this work or i will have to re sell the truck cuz I wont be able to drive it. Please if anyone has any info I greatly appreciate it.

I know of no way to do this unless a body shop has done this before. Maybe change out the seat? There is a safety aspect here when you start adapting things and retrofitting and most all shops will walk away from that.