Three of the Witcher short stories are available online in English.
http://fallout.scifi.pl/The_Witcher.pdf This is an excellent translation by Michael Kandel, the man who translated most of Stanislaw Lem's books to English as well, it was translated for some anthology and later sent out by CD Projekt as promotional material. Unfortunately, Kandel, while otherwise great, took much liberty with proper names - that is, he completely changed most of them, while the translators of the game and the book chose to stick to the original or only change it slightly to make it easier to pronounce. Kandel, for unknown reasons, changed:Foltest to HrobostVizima to KlosthurTemeria to KraVelerad to EthmondMedell to Demellstriga to gombMahakam to ApiphOstrit to OsrughPontar to Fonzorso you need to remember that while reading.
http://fallout.scifi.pl/A_Grain_of_Truth.PDF This one is an excerpt from the translated book, and the translation here is more faithful to the original, but the language used is much more bland (it was translated by Danusia Stok, a Pole, not by a native speaker of English). Kandel would be a much better choice, although they probably would have to tell him to be less creative with proper names.
http://www.sme.sk/datahry/683/shortstoryLesserEvil.pdfA fan-made translation, which was officially released some time ago by CD Projekt. It's pretty full of errors.For the rest of the stories you'd have to buy the books. The first book has been published in the UK and will be published in the US next year. More books are forthcoming. You can read more here:
http://witcher.wikia.com/wiki/Witcher_series