They are sold out.I think any adapter would work. Lots of options.
The GameCube can only do 480p and the HDMI adapter doesn't have HDCP (99% sure).
So almost any adapter should work any tv that supports 480p.
But, Output quality may vary depending the adapter.
Also, here is a list of games that support 480p and/or widescreen:
I have the radioshack device.
It converts hdmi to component and worked on my old bluray player. But I tried on my toaster NES. Does not work. Would not even detect that hdmi 3 was connected.