Kroger's has a generic brand of monistat 3, it's 4%. Adding two bottles of the 2% would not get you 4%, it'll just give you more to mix together unfortunately.
If you have a Freds in you area they sell the 4% MN in the tube just like Family Dollar and Dollar Genral Brand.
It does come in those 3 pre filled tubes just one regular size tube (25g)0.9 net wt.
Any drug store, Walmart, Kmart, and most major grocery stores(more expensive at the GS) should carry it. Just look for the 3 day MN packs instead of the 7 day. All of the 3 day packs I've seen are 4% MN.