I personally use Elos for calcium, though Salifert seems to be within range, as well. I just don't recommend using Salifert for carbonate alkalinity.
As far as calcium supplements go, there are a ton of suggestions. For your purposes, I just recommend one of the simple two part solutions you can commonly find. You could make your own extremely cheaply, but with such a low demand in your tank, I am not so sure the frugality will justify the effort involved. Something like B-Ionic (ESV), C-Balance (Two Little Fishies--TLF), Tech A and B (Kent Marine), and any number of others. I do make my own, however, from the materials sold on bulkreefsupply.com. If I had to choose a commercial supplement, I'd probably go with something like DT's, since they features purer chemicals with no inclusion of "trace elements," which have the potential to build up instead of being depleted. That's merely personal preference, though, as all of those mentioned will work just fine.