Try
www.avsim on their file library. You'll have to register I think, but don't worry, it's not a spam-fest if you do.
You could also try
www.flightsim.com Think you may have to register there too, and it can be hard to gain access sometimes, but it's worth a look.
Specific recommendations, I don't tend to use freebie add-on aircraft, but a few I do have one or two that are certainly worth a look (note that some of them are actually for FS9, but can be used in FSX by installing them in 9 and simply copying and pasting the aircraft folder between sims - doesn't always work however, as FSX uses a slightly different texturing system):
Tin Mouse Boeing 737-200 (note that this is available via a specific forum on Avsim (and is certainly one of the best add-on freebies ever, it easily rivals commercial payware, but I think it might only be for FS9 although you could try the old copy-paste routine in FSX and maybe get it to work!).
All these below definitely work in FSX however:
Harrier GR7 by Adam Preece
Bucker Jungmann by O Fischer
JBK Vultee by Jens B Kristensen
JBK Boeing 247D by Jens B Kristensen
Junkers Ju-52 by Pierino Primavesi
Mustang P51D by J. E. Narcizo/Bob Chicilo
Screenshots of the Tin mouse 737 and the JBK Boeing 247D below (note I have repainted these, they don't come in those liveries as downloaded):
I got all of those off Avsim.com Also note that you can get a lot of very good quality free airliners by joining many of the Virtual Airlines out there, such as WestWind etc. If you want a laugh, try searching Avsim's file libraries for my name (Alan Bradbury) and you will find a couple of freebies, most of them are for earlier versions than FS9, but I do know it all works in FS9 with a bit of file shifting. Never tried them with FSX though.

Chock