At T-Mobile, search for "activation kit" to find the T-Mobile SIM Card Activation Kit
with free shipping
. (Price shows on the product page, not the search results page.) That's tied with our June mention and tied as the lowest total price we've seen. (It's a current price low by $8.) Sales tax is added where applicable. This SIM card lets you use compatible phones on the T-Mobile network. It includes 10 minutes of airtime.
Update: It's now 99 cents with the above shipping, a price low of $7.