there is no point in trying to get better tires for the stock 15inch rims no matter what tires you get they will still suck and look itty bitty the only tire worth putting on them would be some drag radials or slicks.
I can't remember what size tire i have(255 something 15 can't remember the series), but like marv said, go to a tirestore and have them show you what will work. I don't agree on the fifteens are all that bad, i have mine with my winter tires on them, and to make them look better i got white letter tires. Makes the tire and rim not look so bare and dresses them up a bit.