Coin / Currency Collection

My son has started with a coin / currency collection a while ago and his goal is to see if he can collect full sets of coins / currency (paper money) for each and every country in the world. He is also willing to pay for coins etc. At the moment it is very slow as he is inexperienced in sourcing and don't know where to advertise. It would be appreciated if people could assist in sending him coins etc, or let him know where he can obtain it. Thank you.