When I checked the other day, I couldn't actually find the right coroplast online anymore... but I saw it in the store just last week. Don't ask an employee where it is - they'll almost certainly give you a blank look. Ask them where the
plexiglass is - coroplast is always kept with the plexiglass. It's in white sheets... a couple smaller sizes and - hopefully - a BIG sheet (3' x 6'). You want the big sheet - the little stuff is a rip-off.
If your Home Depot doesn't carry it in big sheets, you can get it from a sign shop (they sell it in 4' x 8' sheets and usually offer a wide variety of colors, like the purple in my condo pic above), but the prices are higher (I paid around $18 for a 4'x8' sheet). Failing that, you could just do plywood with something over it to protect it.
How often you need to replace the coroplast depends on how destructive the bunns are - mine chew the sides to bits, but they leave the floors intact (which is what matters). They also pee it up something fierce, but most bunnies aren't so naughty. With really, really naughty bunnies, coroplast may need to be replaced every six months but if they don't pee up the floors and leave residue that you can't seem to scrub off completely no matter what, coroplast could potentially go YEARS without needing to be replaced. It's completely waterproof and can be cleaned with pretty much any bunny-safe cleaner.
You don't actually attach the coroplast to the grids - it just sits on top. This
website shows how to do coroplast floors so that they've got a bit of a sidewall - you don't have to do that, you could just have it flat on the grid floor for each level... however, the side walls help prevent stray pellets, hay and poops from getting flung out of the cage as bunnies skitter around.