I'm trying to purchase Buck roses. Is Northland Rosarium a reliable
mail-order source? What other rose nurseries would you recommend?

You should give Northland Rosarium a try, they have an extensive list of
Buck roses as I am sure you can see. I have had excellent service and plants
with Chamblees in Texas:
http://www.chambleeroses.com/
and Forestfarm in Oregon:
http://www.forestfarm.com/

I have ordered numerous 1 gallons from Chamblees and they all have been
beautiful. Prices are excellent and they are on their own roots.
Forestfarm in Oregon has roughly 55 varieties of rosa, mostly species, some
OGR's and Climbers, also own root. Their variety of shrubs, plants, trees
and conifers is mind-boggling. Check these places out. Hope this is helpful!
Jay