The Sylvania Silverstar's are not that bad of a bulb when compared to some of the other "HID look" bulbs on the market...there are much worse out there. Just don't get fooled by the "whiter light is brighter" marketing claims of some manufacturers.
has the Sylvania XtraVision 9007's listed at $9.99/bulb. Search for Part# 9007XV/UPP
The best place I've found to get the Osram Silver Star & Philips Vision Plus bulbs is http://www.powerbulbs.com
in the UK: doesn't look like either are available in 9007 bulbs though...