If you are looking for Reebock era jerseys, E-bay would be my first go to, all junk though, nothing legit being produced currently.
In case you are looking for new NIKE stuff;
From NFL Pro Shop:
Mid grade jersey, $150 plus shipping: http://www.nflshop.com/product/index.js ... age=family
Authentic, $300 plus shipping: http://www.nflshop.com/product/index.js ... age=family
Currently all sizes available for mid grade jersey, two biggest only for authentic jersey.
Check back frequently as inventory comes and goes quickly.
Hope this helps.