The first of course is Mazda specific, and the second isn't but has a large group of ex MSF members.
Both of these places probably offer a good amount of used MS3 parts, 1st and 2nd gen, and a lot of the guys are both sites are great, with their exceptions, of course.
For MSF, you do have to join and donate to get access to the FS/FT section, their way to weed out the spammers and scammers, no idea how SF.org does their FS stuff, never joined up, since I traded the MS3 in for my Focus.
Once upon a time I had:
2012 SGM SEL Sedan Job 1 BD 6/2011
FSWERKS Cool-Flo w/ ITG Filter | FSWERKS Stealth Exhaust | 17x7.5 Axis Sport XMX Graphite wheels | Eibach Sportlines on Ti Handling Pkg bits