I am not sure if anyone still sells the "Neat Ideas Cubes" brand. I think they used to be sold at office stores? Not sure. Now they come in a variety of brand names. So if you're looking specifically for "NIC", you might not find them.
I know that the Target branded ones are not listed on their website (at least they didn't used to be), but are in the stores. Be aware, though, that the design has changed, so most of the panels in the pack are only suitable for floors that will be covered with another material, or the roof of the condo, or for the side of the cage that is against the wall. (Bunnies can get through the large holes.) The Target branded ones are under the "Organize It" or "Re-Organize" names, and can be found in the home improvement or home organization areas, along with the adjustable metal shelves.
I have read on this forum that some people have gotten them at Walmart, but my Walmarts don't carry them, so I don't know what brand they carry. I would also check in the home areas, they might have some now for back-to-school, with the stuff they're stocking for college students. Same with Costco, I have heard that they sometimes have them, but I've never seen them there.
In light of the design change of the Target brand cubes, I personally think the best value right now (at least in my area) is at Bed Bath and Beyond. I found some in the section with storage boxes and such. They cost a little more, but have more useable panels per pack than the Target ones. Also, if you're on their mailing list to get those 20% off coupons, it can bring the price down. I have seen these only in gray.