Hi,
I do not know which operator you want a SIM for...
Actually you can find/buy some SIM cards at a lot of places... Just keep your eyes open. Bigger newsagents and almost all bigger petrol stations, cellphone shops, large stores like Tesco or MediaMarkt also sell SIMs. Some of these places offer so called 'instant cards', which you can put in your phone, but you need to make a call to a certain number where you have to register by talking to an operator. They will send you a letter in a couple of days to verify your address.
Branded (Vodafone, T-Mobile or Telenor) shops are better in the sense that they register you on the spot. However, you can expect a longer queue/waiting, especially as Christmas is coming. But if you are not very picky, you can can just walk a few meters on and find the next shop, especially in Budapest
I do not know what you are really after, I might be able to help, just drop me a message.