in varna there is only really Erica (that I'm aware of) that is a true garden centre
If you search for “garden centre” in the BG yellow pages http://www.goldenpages.bg/en
you’ll find nine (other than 2x Mr Bricolages) listed in Varna and if you check http://www.need.bg/en
for “garden center” they list four.
Paste градински центрове варна into http://www.google.bg
and, apart from the regular links, it comes up with an A-G list (extended to A-J) of them marked on a map.
If you aren’t aware, Erika has a large garden center, apart from their “in-town” shop (between highways 2 and A2) on the old Sofia road (highway 2), on the way to Devnya, just prior to the Побити камъни. In fact, there are several garden centers out on that road.