I can absolutely, positively, without a doubt, tell you that your best gardening source for Blizzard Treasure Cards is *drum roll* the Frozen Fly Trap! OK, who guessed that the Frozen Fly Trap is the only source for Blizzards? You'd be right, which would be why it is THE best source. However, it is not a bad source, pretty good actually.
While growing 10 plants at a time, I averaged 18 Blizzard Treasure Cards per harvest cycle, which only took about 2 weeks. You should easily get all the Blizzard TC that you'll need to craft the Dragonspyre clothing for Ice Wizards in under a month, especially if you fill an entire large gardening area with seeds instead of only 10 plants like I did.
You'll also need Blizzard cards for crafting Watchtower Hall as well as the Castle Blocks with the stained glass window in them. Best of luck fellow gardeners, have fun! :)