Spire.io
You can purchase Health Tags and the Spire+ Membership directly from Spire.io. Currently you can buy Health Tags in a 1 Pack ($50), 3 Pack ($129) or 8 Pack ($299) from Spire.io.
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 Pack and 6 Packs.
Amazon
Spire Health tags will be available to Amazon customers during the 2018 Holiday Season, pack size and pricing is still being finalized but should be similar to both Spire.io and Apple.