Hebes for cold climates

The genus Hebe has recently captured my attention, and I am wondering if anyone has suggestions for species or hybrids which are very cold hardy. I am in zone 6B or slightly above, and have two species now in our garden: Hebe odora (syn. Hebe buxifolia), and Hebe pinguifolia var. pagei. Both species have been in the ground for about two years, and they bloomed for us this past summer. We would love to add a few whipcord form Hebes, along with some with flowers that aren't white. Does anyone have suggestions of species and/or hybrids which might reasonably be expected to survive here? I suspect I'll have to source seed for plants, as few nurseries in Atlantic Canada have any plants in stock.