Persephone wrote in
:

Thanks, but I went to that Web site before posting. The
URL you offer carries the corners and connectors, but when
I clicked on "composite lumber", it said they no longer
carry that item.

Anybody have another source?

have you checked a local big box hardware place like Home
Depot or Lowe's? if they don't have it they should be able to
get it. one brand name for the composite lumber is Trex.
or try these folks:
http://www.epsplasticlumber.com

i don't know anything about them personally though.
lee