Check out Jófogás, Vatera or Galeria Savaria if you're in Hungary. You can easily find a 1990-2004 standard license plate or even a pair for about 1000 HUF. Sometimes you can get post-2004 or pre-1990 plates too, but they tend to cost more, as well as more interesting variants.

If you want something more special, you can check out the local flea markets. From what I've seen, they often don't really guess the rarity of a plate, which means that you can get very special plates for a great price, but you should also be careful because they may also overestimate the value of common plates.

Feel free to ask if you have any questions. :D