P. montezumae seems to be only intermittently available in the nursery trade here, probably when someone tries and succeeds in getting a batch of seeds to germinate, though there would always be some doubt over what they actually are. I asked the (now retired) conifer specialist at Little Missenden why he didn't grow them, and he said that the Mexicans wouldn't allow export of the seed, because they were protected (a rather silly attitude - surely some controlled export of the seed to maintain diversity in the reservoir of cultivated examples is desirable). But clearly from time to time seed do become available from cultivated sources, or otherwise. And they are more regularly available in the nursery trade in some other countries.

You can try to collect seed yourself from places like Bedgebury Pinetum - they are just allowed to rot on the ground so I don't see why you shouldn't pick some up and try. I think late autumn is the time to go. I did this once, but was not aware of the correct conditions to get them to germinate, and they didn't.

You need to be aware that a lot of the trees in collections labelled as P. montezumae are actually P. hartwegii. These will also cross easily, so the seeds one collects may be the cross even if the tree isn't. P. hartwegii (the correct modern name) was previously known as P. montezumae var hartwegii, so the label may actually have been "correct" at the time it was applied. Another superseded name for it, commonly seen on arboretum labels, and used in the nursery trade, is P. rudis.

Given the level of confusion, you may discover that P. hartwegii alias P. rudis is satisfactory to the customer, indeed it may actually be what they have seen and what they want, and may be easier to source. I believe they are a bit hardier and easier to grow.

P. Engelmannii, or Apache pine, is a similar tree, even more beautiful long-needled pine, which I have seen at Wakehurst Place, and it seems to be even harder to get one than P. montezumae, though at least the seeds are traded.