Is the item still in print? There's not one single site, the distributor, or label, could be a good reference as long as the item hasn't had a SRP reduction. Otherwise, I attempt to go back and find a press release for the item. You can also try using one of the Alliance Entertainment listings if all else fails and the item is still in print.

I avoid using Amazon (and Blu-ray.com by extension since they just use Amazon data) since Amazon is notorious for having wrong SRPs or making up their own with their price fixing.

If you mention the title, I could attempt to see if I can find the original SRP too for you.