Some good H4 adapters can be found at www.casperselectronics.com you may have to call because I don't think they list the part on the website. The shield they sell is the best one I've seen yet though, and is worth getting. Caution though, the adapters only work with Philips bulbs - not with OSRAM bulbs. So check what you got before you order. IS300s can come with either brand!

Another option for you is to put the IS300 HID reflectors into your assemblies. It's very do-able and has been done before. That way you will get an OEM quality beam pattern, the nice color you so desire and you won't annoy other drivers too much. It's extra work, but the result will be better than just sticking the adapted bulbs in there.