Far as I know there's only two types. both 21 watts. The difference is in the positioning of the pins which hold them into the bulb holders. One type has the pins directly opposite each other - so one pin on either side of the cap. The other has offset pins.
Oh, hang on! I think the 500 uses "capless" indicator bulbs. If that's the case there's only one to choose from so just look for them on the shelf at Halfords, local factors, or any one of a multitude of suppliers - I've even seen them in Aldi/Lidl. Any motor trade source will have them.