I have worried a lot about curriculum. Like, A LOT. I know I should be praying more and worrying less, but the Caldwell genes are strong with this one. (PS We're worriers.) What's weird is that I haven't worried so much about pre-K curriculum, I am "borrowing trouble" for kindergarten. Pre-K has been an easy decision.
I am part of several Facebook homeschooling groups and they pretty much unanimously agree that no real curriculum is necessary for pre-K. You can get preschool workbooks just about anywhere. Walmart, Dollar Tree, etc, but my favorite place is Ollie's Bargain Outlet. If you don't have an Ollie's, think of it as like a cleaner more organized Big Lots. The best part of the whole store is the book section. It is huge! There are tons of kids books under $5! You can get giant workbooks for under $5, too! I just supplement with bible stories. I could pay for a Christian pre-K curriculum, but for ages 3-5 I do more teaching from just walking down the street pointing out things than from a book anyway, so why waste the money?
I never know what I will find, so I stop in every time I'm up that way. They even have educational computer games!