Seed Companies
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World Record Giant Variety Seeds (USA) If you are serious about growing giant sunflowers and giant vegetables than this is the place to go.  Carry some of the best varieties for breaking records that you can't find anywhere else, such as Diane Strain, Craven, & Hemker multi-headed.
International Seed Distributor (CA)
Largest selection of imported seeds that sells to the USA.  Carry Unwins (Giant Single), Suttons (Giant Yellow), Mr. Fothergill's (Giant Single, Titan),  McKenzie Seeds (Large Seeded Tall), Thomspon & Morgan (Russian Giant), and more.
Mr. Fothergill's Seeds (US & UK)
U.S.A Distributor for Mr. Fothergill's Seeds (Titan, Giant Single)
United Kingdom only: (Titan & Giant Single)
(Paul Bunyan, & new Cyclops Hybrid)
Thompson & Morgan (US & UK)
(Russian Giant)
Renee's Garden (USA) (Sunzilla)
Park's Seeds (USA) (Giganteus)
Ferry Morse (USA)
Unwins (UK)
United Kingdom only (Giant Single)
Suttons (UK) United Kingdon only (Giant Yellow)
Chiltern (UK)
(Diane's Strain)
chiltern seeds
VidaVerde (UK)
(Bott's Strain)