Portables usually go in the range of $500 and up Currently I don't know of anyone selling their portable, but you could always build your own. We have all the guides you need to get started in the Guide Hub.
Yes, you gotta find listings, or offers, you could check places like craigslist, or wait for someone to sell one on here.
you can also commission one, though I assure you, asking for a gamecube portable is going to cost more than a wii portable, because a gamecube portable requires old parts, that are hard to find, or obsolete.
so i'd commission a wii portable instead of a gamecube, it's your money, do what you want with it, but that's just my opinion