After some more research and long discussion with a guy at Autozone I found that mini bulbs basically come in two flavors: 5 watt and 3.5 watt. As long as you get the right wattage they are interchangeable.
So, for example, in my case the W5W9Q4, the "W5" means it is a 5 watt bulb, so I can use a 5W mini bulb. I used a 2821 Sylvania mini bulb which looks pretty much identical to my bulb and worked well.
I could have used a 3.5 Watt bulb, but it would not have been as bright.
The main mistake you want to avoid is putting a 5 Watt bulb in a plastic housing designed for a 3.5 Watt bulb, because you could melt or discolor the housing or lens.