Exceptional pumpkin variants Five or six years ago, I happened upon a pumpkin in the bin at a Whole Foods in Cambridge, MA, that turned out to be substantially different from the others I've had.  It was, I think, about eight pounds.  Its skin was milky white with very faint, thin green veins, and its flesh had a distinct reddish tone to its orange color, very different from the usual pale or medium orange of a pie pumpkin.  After baking and pureeing it, the resulting mash was unusually dark and dense and the pie I made out of it was fantastically rich and flavorful.

I have no idea what kind of pumpkin this was.  I suspect that the store was using it only for decoration, or to be sold as a decorative pumpkin, because it was in no way advertised as a pie pumpkin (it was way better than any pie pumpkin I've had) and the employees weren't even sure how to ring it up.  Other white pumpkins I've bought that I thought were the same turned out to be no different from similar-looking orange pumpkins.

Was I the lucky beneficiary of a unique gourd or is there a particular pumpkin species or cultivar that displays these properties, and if the latter, what is it and where can I get more?
Based on your description and some of the feedback from comments, I think the most likely candidates for your mystery gourd are white pumpkins of the Cotton Candy or Valenciano varieties, possibly also Lumina or Silver Moon (though those tend to be flatter and larger than you describe).  Here's a link with some further comparative description.

The dark color of the flesh that you describe was more likely a product of the growing season than anything else, though you might have some luck asking farmers at local pumpkin patches what specifically could cause that result.  How you cooked the mash could very well have contributed too.  Here's an excellent post from the community blog with instructions for producing quality, richly-flavored "orange goo".

Should you be completely unable to find a similar specimen, you may need to consider substitution.  Possibly the best one I can think of is butternut squash.  This has a very rich flavor, quite similar to pumpkin, and it gets richer as it ages.  Look for ones with dark skin - the darker the outside, the darker and sweeter the inside will be as well.