The best way is for you to save your own seeds from cultivars that succeed for you in your own garden. Next find a seed company that grows its own seed (not all do) in climate with a growing season like yours. Watch for micro climates which is quite trick in mountains and coastal areas.
While heirloom really mean that that particular strain was saved by a particular gardener or family, open pollinated is important as it you can save your own seed. You wont find much GMO seeds outside of commercial farm seed suppliers. Organic seeds are always good but not vital, IMO although raising they organically yourself is definitely worth pursuing for yourself.
Become familiar with the vitality longevity of each cultivar as the is no point in buying big quantities parsnip which only is good for 1 year.