Formula dilemma

My LO is breastfed and I also supplement with similac pro advance ready to feed bottles because I don’t produce enough breastmilk. Well currently there is a shortage of the ready to feed bottles. Can’t find them anywhere including online. We are working on transitioning to whole milk or another alternative but so far we haven’t been successful. In the meantime until we can fully transition to whole milk, do you think it would be ok to supplement with the powered formula? I’ve heard it could possibly cause an upset stomach. Just afraid to switch to something he’s never had before and cause tummy issues.