A couple of years ago, I saw a plasma/lightning ball for sale that was shaped
like a regular light bulb and screwed into any standard bulb socket. 
I'd like to know where I can buy one.

These things used to be plentiful, but apparently the fad has crested.
Although larger plasma globes are offered by many merchants, I have
found only two sources for a plasma bulb that fits into a standard
light socket. Both sources are eBay merchants:

"This listing is for a brand new Sunbeam 4 Watt Plasma Bulb.  It fits
in any standard 120V 60Hz lightbulb socket (base not included). 
Electrifying currents dance to your touch!  This is a great deal! 
These lights are a real novelty!"

eBay: Sunbeam Plasma Bulb 
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=43424&item=4380279391&rd=1&ssPageName=WDVW

"this is a sunbeam plasma light bulb. it is a lightning storm in a
light bulb. these are brand new in the box and are real cool... they
will fit in any lamp socket and are great entertainment for kids."

eBay: sunbeam plasma neon electric light bulb
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=371&item=6178785217&rd=1&ssPageName=WDVW

I hope this will meet your needs! 

Best regards,
pinkfreud