Please Please Please, if youre going to a country and you cant buy the currency here in the normal fashion. ie banks, post office, travel agent. Dont assume you can get it on ebay.
Countries where money isnt availible here have whats called a Zero Export or are called Closed Currency. Meaning its illegal to take it in or out of the country (Most of Africa have this law).
Also, where a currency has just become availible (ie Bulgarian Lev, Brazil Reals) theses in the past have revalued and the old notes are still about. Try and keep an eye on the date on the notes and make sure they are recent.
Best advice is. Follow that at your bank or travel agent. If in doubt take GBP£, Euro or US Dollar.