It's best to swap them in pairs to ensure colour match. The OEM ones will have fadded with age, so even if you fit a genuine one, it will be a bit brighter. Plus, if one has gone, chances are the other won't be far behind.
Get on the internet, buy a pair of bulbs and fit them yourself. It's only screws and clips. Even the mechanic had to figure out how to do it the first time.