Ok might not be what OP wanted to know...but "apart from buying or trading with other people" sounds like someone doesn't know about the Toy Vendor, maybe?
If so then you can buy boxes of unknown cubis from them and open them to find out what's in the boxxxxx!
During the 1st Beta I bought them from the Toy vendor and was lucky enough to get many rare ones. Since then the only ones I have bought were for Gifts. I do, however, have more than will fit in my Cubi bag because I have been gifted several of them and have found a few others (mostly pigs, chicks and butterflies). When I find them, I gift them to new players or to decorators or use them at parties. If you have no cubis, you will enjoy the piggies, chicks and buterflies.