By exterior courtesy light, I assume you're talking about the lights that turn on for a bit in the composite headlights after opening a door? There are two--the upper amber bulb is 3157NAK, the middle side clear bulb is 12V5W W5W. Aftermarket bulbs will have slightly different nomenclature, but searching using those part numbers will give you the right bulbs. You should be able to buy a replacement bulb at your local auto parts store, they're very common bulbs. When replacing it, just wash your hands first and avoid touching the glass bulb so you don't get oil on it. I use a clean paper towel to handle the bulb.
2011 Ford E350 EB Quigley 4x4 'PUPLGUK' | V10 Gas | Opt Overland Pop Top | GBS LiFeMnPO4 100Ah