I do not know about the seahorses, but I have regularly seen clams at Aqua Imports in Boulder and also at Animal Attraction in Greeley. The ones at Animal Attraction are under a few smaller LEDS that are a few feet above the clams - I would worry that these have been slowly starving if they have been there for long... if they just got them in stock within a week, or so, then they might do better.
I have seen a few clams at the one on Harmony in FoCo (forgot the name), but not consistently - I have the same concern for long-term health under the AP700s at that much of a distance.
Clams are not something that you want to sit around the LFS for long - right out of the ocean is best unless the shop has them very shallow under a TON of light.