When I'm looking for high-res sponsor logos, I find that usually the best place to look is in PDF files on the company website. Look for a quarterly report or something, which will usually have a high-res logo in it somewhere (seeing as it's meant for printing). Then I usually just take the cheap way out by zooming all the way in on the logo and taking a screen capture. Sometimes you have to change the colours later.
I know this is no longer relevant to this particular thread, but I thought it might be useful for future reference.
-Ian Mackenzie
Team 188
Woburn Robotics