The hold down part is called a "pip pin". They make
a convenient quick release mechanism for double-shear
loads. As an aircraft tiedown used in shear, pip pins
are misused. I have pip pin tiedowns on my 235, but
every time I tie the aircraft down outside, I hope
the wind doesn't blow too hard, because I know the pip
pin isn't going to take the kind of tensile loads the
wing might generate in a storm.

Pip pins are also used to hold the space shuttle down:

http://spaceflightnow.com/shuttle/sts092/001010scrub/

Anyway, here's one source....
http://www.pegasusautoracing.com/productselection.asp?Product=3050

Another might be Lancair Parts, but I didn't see it on the website.
http://www.aerocraftparts.com/