Craft Beer aluminum can label bandaids. The labels are super sticky, water resistant and work great on skin for blister prevention and relief. Cut to size.
The best blister prevention (or if it is too late and already a blister), regular old duct tape. Don't get the Dollar Tree stuff though, just a good middle of the road brand. The super spendy stuff works good, but might pull at the skin a bit for removal. Been using it for about 15 years now when I know I'm putting on 10 or 15 miles in a day.