I've been using 3 ounce-or-less tubes for years in my travel kit.
Pharmacy is where I usually buy it. In addition to the above-mentioned Congate and Aquafresh, more commonly one will find specialty brands that cost more (e.g., $3.50 for 2.7 ounces of Topol).
No matter, I like to travel with a stain-removing paste because I tend to drink a lot more coffee on the road.

Brands noted earlier this week when restocking my kit, er, baggie:
Topol comes in 2.7 ounce, and tends to be the least costly
Rembrandt, 3 ounce
Plus White, 2 ounce tube and 3 ounce gel
Burt's Bees, alas, comes in 3.2 ounce
Pearl Drops, 3 ounce liquid
Assorted 2 to 3 ounce containers of children's toothpaste/gel, for your little one's baggie.