You can buy Sunflower grade Voronezhskijj 638 in online stores (seed and seedlings).
$8.49 David's Garden Seeds Sunflower Big Smile D1312 (Yellow) 50 Open Pollinated Seeds
$20.39 Kaytee Sunflower Hearts and Chips Bird Seed, 8-Pound
$11.29 Kaytee Sunflower Hearts and Chips Seed, 3-Pound
$5.95 CRAZY MIX Sunflower Seed Mixture - Over 12 Varieties of Sunflowers - Showy Flowers, Extended Blooms, Easy to Grow - By MySeeds.Co (250 Seeds - Pkt Size)
$9.99 Garden Starter Kit (Mammoth Sunflower) – Grow sun flower seed in a mini greenhouse, then plant a beautiful patch of Sunflowers in your backyard. It’s easy, fun, and a great gift for adults and kids.