Spire.io
You can purchase Health Tags and the Spire+ Membership directly from Spirehealth.com. Currently you can buy Health Tags in a 1 Pack ($49), 3 Pack ($129) or 8 Pack ($299) from Spirehealth.com.
Apple
As of September 2018 Spire Health Tags are available in Apple stores and online at Apple.com, pack sizes may vary based on Apple store region or country but in general you will find them available in 3 Packs and 6 Packs.