Frost Flowers can hail from Boon Trees, Evil Snow Peas, Fickle Pickles, Frozen Fly Trap, Honey Bee Plants, Orange Dandelions, and Snow Apples. Unfortunately, I don't have any information gathered yet about Boon Trees or Snow Apples.
Evil Snow Peas - 3 per day*
Evil Snow Peas have a very short cycle. Even without stuff they like, it only takes 5 days to go from seedling to elder harvest. Growing 10 plants for 10 cycles, I received 141 Frost Flowers in 49 days. The average cycle will produce 14-15 Frost Flowers every 5 days.
Evil Snow Peas seeds are sold by Farley, in Golem Court, for 1800 gold. I asked Windbreaker and he says he knows of 87 creatures that drop Evil Snow Peas seeds.
Frozen Fly Trap - 1 per day*
I grew 10 Frozen Fly Trap for 5 cycles which took 75 days. During that time I harvested 70 Frost Flower, That is an average of 15 days per cycle and you can probably expect to receive about 14 Frost Flowers per cycle.
Frozen Fly Trap can be bought in Khrysalis from Barley, from Harley in Celestia, or from the Zafaria gardener Narley. These seeds will cost you a whopping 3000 gold each. Windbreaker says you're better off hunting for them rather than buying them, there are 89 creatures that drop Frozen Fly Trap
Orange Dandelion - 1 every 2 days*
Orange Dandelions take an average of 10 days from planting to elder harvest. After growing 10 plants for 30 days, I received a total of 16 Frost Flowers. On average they can produce about 5 Frost Flowers per cycle.
Orange Dandelions are sold by Farley, in Golem Court, and Roger the Shrubber, in Avalon, for 1200 gold. When pressed for information Hunter revealed he could think of 65 creatures that drop Orange Dandelion.
Honey Bee Plant - 1 every 13 days*
Honey Bee Plants aren't the greatest source of Frost Flowers, but you CAN get them. The Honey Bee Plant takes 25 days to reach elder harvest and gave me 2 Frost Flowers, which about average for this plant.
You cannot buy Honey Bee Plants for gold, and Hunter only knows of 19 creatures that drop Honey Bee Plant but admits that there are many others that he doesn't have information on yet.
Fickle Pickles - 0
Fickle Pickles are reported to drop Frost Flowers. I have grown two cycles of Fickle Pickles, which took 30 days and never received a single Frost Flower.
*Based on average number of reagents received per day of effort put into gardening. Obviously none are received before a plant's mature stage.