Should be available for sale during the planting time I suspect.

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.