Someone's already suggested Wacom; I'd get a bamboo, they're quite affordable. There's a company called monoprice that's worth looking into too, their stuff's been getting really good reviews and it's pretty cheap in comparison. Haven't tested those myself though, so I can't be sure.
Be forewarned: tablets are never an 'Easy transition' from traditional art. The hand/eye/screen coordination takes weeks to get used to, and you'll never have the same exactitude of coordination you get with traditional art. Use large image sizes and make frequent use of the zoom in/out tool; drawing zoomed out makes the tablet's line less precise.
You'll get a good beginners graphics tablet for under a hundred. The Sai license cost $64. There isn't a Mac version of it however if that's what you are using. Any stylus is more analogous to traditional drawing since its pen like I think. You just have to get used with it and get used to look at the screen while drawing on the plate.
Graphics tablets with a screen and the kind of pressure sensitivity and stylus control that lends itself to drawing is still quite expensive.
Wacom tablet all the way Since you're a starter, I'd recommend Bamboo. It's a quite simple tablet but it's got all the pen capabilities you will need for painting, drawing, etc. And it's pretty cheap too.
Paint Tool SAI is very good for drawing manga, if that's what you're into. It's a Japanese software, but I think there's a free English version you can download somewhere. Just google Paint Tool SAI and you'll find it.