A few of the old AMT semi kits are being re-issued, the Chevrolet Astro earlier this year, a Freightliner COE and International conventional later this year. Hopefully they will be doing a few more next year too. They have also re-issued some of the trailers.
Model round up still has some of the Stevens and Model King reissues from past years, a couple Macks, an Autocar, Ford C900 and Louisville snowplow.
http://www.modelroundup.com/
STS has books, kits and aftermarket stuff for truck models
http://www.stsmodeltrucks.com/
AITM is a good aftermarket source particularly if you like older trucks
http://www.aitruckmodels.com/
Sourkrauts Model trucks is another
http://sourkrautsmodeltrucks.com/
KFS has some nice stuff, their chain binders are nice for a flatbed or low boy trailer.
http://www.kitformservices.com/
I've bought stuff from all of the listed venders and have always had good service.