If you have a garender that always have everything that they need in their garden, then a gift certificate would not be impersonal since the gardener will use it for additional flowers that see in the store.
One can find good gardening supplies at their local hardware store. Walmart and Target have the best prices but their in-store options are limited.The website "Gardeners" would be the place where a person could find everything related to gardening and gardening supplies.
I would try a gardening store, like Star nursery. :)
The Gardening store
One of the first places I would try is your local library,there are many books on gardening there. You can also go to a local gardening store and ask a clerk there for advice, many clerks in these stores know quite a lot about gardening
at your local gardening store.
Your local gardening or handyman store.
SmallBoxGarden is an online store where you can get a variety of garden supplies and tools at reasonable prices and begin any form of gardening you like, whenever and wherever you want. The store is motivated by a goal to help people live healthier, more active lives with happier homes via gardening and to make home gardening accessible to everybody.
Greenfingers is the leading UK online gardening store and resource offering gardeners advice and gardening tips.
Your local lawnmower and gardening tool store would have that, or be able to get one in easily.
You don't need a certificate but if you want one you can get one from a local computer store.
in the store
A humidistat. (Yes, you can buy them in stores ... try home improvement centers, pet store or gardening centers.)