You won't find that kind of stuff in common stores, you'd be lucky to find them at a game or electronics store even.
Kyosho is the first person I've heard who actually likes the redant box.. you might want to read reviews of it first. The main thing to consider is that it doesn't support progressive scan at all. It has to convert (or "upscan") interlaced video to make it work on a monitor. I haven't seen the redant box in action for myself, so I can't outright condemn it, but upscan conversion usually gives a really shoddy picture.
egman mentioned VGA cables made for the GC, and that's the solution I went with. Unfortunately you can't use it with non-progresseive games, but I've only played one of those anyway so... I'm not complainin'. All first and second party games support prgressive scan, as do most third party games of any consequence.
I made the cable myself, out of a component cable from Nintendo, a VGA extension cable, and many hours of tedious work. If you search a little you can find several places selling these things pre-made, and fifty bucks doesn't sound at all bad.