I believe you already know that Ledger Nano S is meant for storing your bitcoins safely and not to buy/sell. A few options for buying: Coinbase, Kraken, Bitstamp, Bitfinex (KYC verification needed while dealing with fiat) assuming that you are either from Europe or the U.S and you are planning to use Bank account or credit cards to pay. If you want to use PayPal, Skrill, Neteller etc. then: Virwox, BitPanda, PayBis (no verification needed in most cases)
Exactly, Ledger Nano S and almost every other ledger are just your depository, your safe to store your crypto currency. For continents other than Europe and US use PayPal as it is the most common to be used.