The bolt pattern is 4x100 (the same as a great many cars, so you can find used wheels anywhere. There are a few VW wheels that come to mind that would look nice on a Yaris, plus anything that fits on a civic (older ones) will fit on your yaris). Offset is the problem, have a look here,
http://www.toyotanation.com/forum/t105446.html where they talk about it.
That said, if you find a used rim, see if you can arrage a test fit before you buy to see how they're going to sit on the car. If you're buying a new rim, try and find someone who has the same size and offset wheel to try out on your car so you can see what it'll look like.