They are quite plentiful. Best Buy, Wal-mart - pretty much any place that sells digital photography stuff.
There are different brands available. . . Some folks swear by one brand and others swear at them. I use San Disk. For my house scanner I have a 32GB card and for my car scanner I have 16 GB. They'll run between $18 and $30, but I imagine they can be found even cheaper.
Most of them come with the adapter cards for the HD size. Many newer computers have slots where you can plug these cards into the computer and do your programming that way.
When you first get your card, you can re-format it using the SD Formatter utility.
https://www.sdcard.org/downloads/formatter_4
Then use Sentinel to Clear User Data. That installs the needed files on the new card.
All of these steps are fairly easy.