Best bet is to get some over the counter orthothics and find a sandal with a back to it so they'll stay put. Fly London do loads, but they're a bit marmite -y - but if you do love them, that would probably work!
See a podiatrist who specialises in this and get an orthotic prescribed. If you don't get it sorted you could get back or hip pain . And it doesn't matter how nice your footwear is if you are in pain .
Once you know what you need, you might be able to get future ones OTC. I have a teen who is a shocking under pronator and this has worked well for her
There is also a brand called Vionic which are good but some are very old lady.
I got proper orthotics prescribed and wear them in shoes but they are harder in sandals obv. I seem to be ok if I wear the orthotics and/or fitflops and other properly supportive shoes the majority of the time. I can then get away with wearing something nicer for short times.
If you google shoes or sandals for over pronation you will find some suitable.