IN the past, i have seen some map/battle mats cheeper via vendors at these cons, than in stores (or online)..  Anyone know which vendors usually have them?

I've purchased battlemaps from Chessex.  They have a few they bring each year that are "ding/dent," but honestly, they are in GREAT condition and good prices.  I bought a skirmish, regular, and even a mondo mat and I'm very happy with all three.  For example the regular size mat has a printing smudge that, had they not pointed it out, I don't think I would have noticed to this day.

As far as printed maps such as forest, caves, or whatnot, I could not say.

The Gaming Paper guys have some great items for sale.  Check them out

I completely forgot about Gaming Paper.  I've purchased their product at Gen Con in years past.  For disposable maps, they're great. 

I've used their product to RPG's I've ran at Gen Con and local events.  I like that I could prep the maps and then trash them afterwards, not needing to pack them to take home...one less thing to carry.

I've even received a package in the mail from a gaming buddy who wrapped the box in his group's used maps.  It was funny to see the dungeon map on the package.

I don't know if they'll be at Gen Con this year, but I remember them having deals on multiple rolls at past events.