It's not likely that the tag will be in range and active all the time. They only work at a very short range and are only supposed to be active for a short time - I once accidentally left the phone on top of a tag and it nearly drained the battery in a matter of hours! It's more realistic to expect that the tag ID will be read, matched and then the signal will be lost. Toggling actions is a very good fit to the expected use of such tags (as per the Sony smart tags) simply because of the way they work...
I got my tags from EBay as I only wanted a few. A good website is www.nfcdog.com and it seems as though my tags came from their EBay shopfront (nfc-supplies). Mine are the MIFARE Classic 1K tags as I wanted a larger capacity to play with. If you're only using the tag IDs then the smallest and cheapest tags you can find will do just fine..