I have a good hungarian friend, and he tells me Budapest is flowing over with weed, but that you need to be a bit careful who you buy from, and buying off the streets can be hard.
Apparently, scoring bud is easier in the suburbs, and he espescially recommends Biatorbágy. Find a group of 20 year old hungarians, they are all very friendly, and just ask if they know how to go about scoring some weed. Most are poor English speakers but theres usually at least one in a group fluent enough to help you out.
If you dont want to venture into the suburbs, you are stuck with buying from the gypsies more or less.
You also have the partyboats, with one being very infamous for its availability of drugs. It's much easier to find hard drugs than weed there tho. The boat is in the docks on the island between Buda and Pest. Go there Friday os Saturday evening, but come rather early as supplies are limited.
Finally, weed quality in Budapest is generally quite good, at reasonable prices. Expect to pay about €8-10 pr gram for mid-high quality.
Ofcourse, the Czech Republic is not too far away, and if you dont mind doing a bit of smuggling you should be able to buy better weed much cheaper there.