There are many stamps that depict/commemorate emergency medical services. However, those that show an actual ambulance are few in number. I assume you already have the US stamp (#2128) from the Transportation Series. Sorry I don't have scans of them all, but here are a few more showing ambulances (Scott catalog numbers)...

Australia 1602-1605 (I think #1605 shows the back end of the ambulance?

Austria 1201

Alderney 239-244 (27c stamp shows the ambulance)

Norfolk Island 534-537 (536 75c stamp shows ambulance, sorry I don't have a scan of that stamp)

St. Helena 641-644 (644 60p stamp shows ambulance, sorry I don't have a scan of that stamp)
This list is not comprehensive, but just to let you know what to look for.
There are a other stamps that commemorate EMS but don't show ambulances. Now here's a "fun" one. The EMS doesn't stand for Emergency Medical Services; rather Express Mail Service!

Thailand 1138
Good luck hunting for these!