Where do currency exchange dealers source their foreign currency inventories?

Money changers can get currencies from two of the following sources (needless to say this answers varies from country to country):

1. Designated Commercial Bank for each FCY in a particular country. For example in UK, the designated US Currency Bank could be Barclays. Thus Barclays would have an arrangement with the US Banks like Citi or JP Morgan Chase (in the US) to physically take possession of US Dollar notes and have them securely flown into the UK, where they would further be distributed/disbursed.
2. Central Bank - rather than using a designated currency bank, the central bank could be in-charge of all such physical procurement of foreign currencies.