Walmart offers the Straight Talk Bring Your Own Phone Activation Kit for 98 cents. (Amazon has it for the same as an add-on item.) Opt for in-store pickup to dodge the $5.99 shipping charge. That's the lowest price we could find now by $3. It contains CDMA dual and nano SIM cards, and activation instructions.
Note: This kit is not compatible with GSM or AT&T phones.