Are there any nurseries that sell Crocus pallasii, Crocus goulimyi or crocus sativa in the US and do these grow in zone 5?
I would be surprised if McClure and Zimmerman were not offering them, because they have such an extensive list.
I got crocus goulimyi from Brent and Beckys bulbs last year. They are really very nice bulbs. The variegated foliage was a big plus.
All of those Crocus species should be hardy in Zone 5, plus Crocus speciosus, Crocus banaticus, Crocus kotschyanus and other species and cultivars. Crocus pallasii may be very difficult to find, as a quick search did not turn up any US vendors. All of the others are fairly common and should be available from multiple vendors. A web search for Fall Crocus will list many of them. Before ordering, sure to check out any selected vendor at GardenWatchDog.
You may also be interested in other fall flowering bulbs that can be hardy in your Zone, such as many of the Colchicum species and cultivars and Lycorus squamigera.
Two others that can be easily grown in pots and moved indoors for the winter are, Sternbergia sp. and Rhodophiala bifida.
I would suggest you take a look at Odyssey Bulbs (www.odysseybulbs.com). I don't know which species are hardy in Zone 5, but I do know everything I've gotten from Odyssey has proven hardy to Zone 7.
Be aware that C. sativus seldom blooms a second time unless conditions are just right: lightly fertile, perfectly draining soil in a semi-arid Zone 8 climate. Some years I get lucky and they rebloom, most years I don't. I'm especially partial to C. speciosus and its many varieties, myself. Colchicums, with a general rule of the less expensive the variety within the selection from a reputable supplier like Odyssey Bulbs or Brent and Becky's Bulbs the more vigorous the plant, will also do well for you.
Crocus sativus flowers very late here, just about time for frost. There are growers in Lancaster County , PA with large areas planted. They collect the flowers, then sell the saffron. My bulbs flower every year. I planted them on a hot, dry western facing slope. One of the growers I visited had slightly raised very large rectangular beds in full sun. The bulbs are lifted and divided every few years.
If you have rabbits around, they will eat all the foliage unless you spray or cover the plants with wire mesh.