I get my 31.45% at Lowes (Trop/Ft.Apache). Used to be ChemTek contractor grade MA but lately they’ve only had the 14.5% so I switched to CROWN. It’s in the cleaning supplies aisle on the floor under the shelves - not with the pool supplies. Not sure it’s the best price but I get the 4-gallon case and I’m set for a while so it works for me.
I can only speak for the Home Depot on Decatur & Trop and I can never find any 31.45%. which sucks ‘cause that’s a more convenient location for me.
But do realize you should store it outside. Not near any metals. I use a 20 gallon plastic trash can with lid to put the MA bottles in. If you have children, you should find something you can lock/latch.