Heather is the common name for Calluna vulgaris, nearly all of which are late summer into fall (sometimes later) bloomers. Heath is the common name for Erica species, many of which have late winter into spring blooms with Erica cinerea blooming summer to early fall.

Not sure how they are presented where you are, but nurseries in this area offer them when they are in their bloom season, the exception being those that offer seasonal foliage color - a nonblooming heath or heather is unlikely to inspire any impulse buying, most being a rather undistinguished green lump.

Most of what should be on offer right now are the Ericas, so you can select by bloom color. Also look for Callunas with seasonal color, like 'Spring Torch', 'Moonglow', 'Ruby Slinger', 'Flamingo' or 'Arina', with new growth tips in shades of cream, yellow, pink and red - this can be almost as attractive and colorful as the flowers. In our zone 8 climate, you can plant them whenever you buy them.

If you're interested in a highly reputable American nursery, here is one west of the Rockies (to save on shipping). Well regarded by W. Coast lovers of heath and heather, their catalog gives valuable info on growing and caring for h&h.

Here is a link that might be useful: Heaths and Heathers