According to
sytyauthority.net, the GM number is 15637803 if it's still available. You should also be able to use a CST #7513 or a Stant #11227. I recall that LMC truck had them also for the low, low price of $110 if money is no object...
Dunno if you can get a lower pressure one or not. Personally I don't see the need for one since our IC's never get *that* hot. A lower pressure cap would allow it to bleed off pressure sooner in the event that it did get hot though. However, it would lower the boiling point slightly since the boiling point of a liquid is governed by heat and pressure. That's why water boils at 212 degrees at sea level (high pressure). At 68,000 feet, it boils at about 98 degrees (low pressure)...