Thornhayes Tree Nursery and Burnham Orchid Nursery, both in Devon, join members such as Burncoose Nurseries and Duchy of Cornwall Nursery in Cornwall, Charles F Ellis and Caves Folly in Worcestershire and Lazy S in Monmouthshire.
The guide says it offers a new way for specialist nurseries to promote themselves at flower shows without actually being there themselves. Founder Gilly Pollock said the company's Morris Minor Traveller attracted a lot of interest from the public at the Malvern and Cardiff shows.
She added: "The public are really keen to support specialist British nurseries but they often struggle to find them. We have had a huge amount of interest and it is just starting to take off."
Pollock said she was hopeful that the company will be exhibiting at the Hampton Court Palace Flower Show in July.