Yeah, they cost something but Sai is really a god painting program and worth the money (compared to other programs = cheap and effective) as well as a small bamboo tablet. You don't need to have the most expensive one, but I really can recommend the tablets from Wacom. Using an older Wacom Bamboo Touch Tablet, Size A5. It's probably not the best one, but it fits my needs.
If the tablet of yours fit your needs you don't need to buy a new/other one and instead of Sai you could try your luck with GIMP (freeware, I think). Never used it but it seems to be the most common program for people who don't want to spend any money for their programs :/