Hey warbirdsnut,
First place to look for old big props is CalClassic.com. This is passenger and freight propliners' heaven.
Besides what you'll find there (a.o. Boeing 377 Stratoliner, C-46, Convairliners, DC-6, DC-6B, DC-7, DC-7B, DC-7C, Connie, Starliner, Electra, various brilliant panels, propliner tutorials, scenery) you'll find links to relevant sites et cetera.
Jens B. Kristensen has made a beautiful Canadair DC-4M & C-54GM North Star and C-4 Argonaut package (combines wonderfully with Des Braban's Lancaster sound), ditto Douglas DC-4/C-54 (my advise for sound is Des Braban's DC-3 sound in combination with DC-4 sound.cfg by Maarten Brouwer :redf
, and ditto Douglas DC-6 (my advise for sound is David C. Copley's Sim-Outhouse A-26 Invader sound in combination with DC-6 sound.cfg by - again - Maarten Brouwer :redf
.
MAAM-SIM has a fabulous "payware" DC-3/C-47 with a wonderful sound pack (based on digital recordings of Douglas C-47A PH-DDZ's Pratt & Whitney R-1830-92 engines).
Diego S. Barreto made some really good Wright Turbocompound sound packages for the Douglas DC-7 series and the Super Constellation/Starliner, based on sound recordings of the real aircraft.
Avsim.com, Flightsim.com and Simviation.com e.g. have huge stocks of repaints, sounds and so on.
Good luck.
Cheers,
Maarten