If the cushions will get wet you may want to invest in marine foam.

Here is a link that might be useful: Marine foam

We have a place in our area called The Foam Factory. I haven't been there but they do sell nationwide and carry outdoor foam.

Here is a link that might be useful: The Foam Factory

Marine/outdoor foam + sunbrella fabric

Thanks so much!

These will be on a covered porch, so no direct exposure to rain, although we do have our humid days. Do you think I could use regular upholstery foam?

Foam Factory is one of the places that I was browsing. They have a good selection and decent prices. The internet reviews are mixed...apparently it's not the best place to buy a latex mattress topper, but they have good reviews on all of their other products.

There is a special type of cushion material in fabric stores that is made for outdoor cushions. It is more like a super bonded poly batting and claims to be very durable, not mildew, etc. You can cut it with scissors. Wish I could remember the name of it, but it comes in large rolls, or by the piece. I used it for a window seat once and it worked out well.