the last tubes I got were from Landry's in Natick, MA. they have a good selection of Schrader for 700 wheels including long stem if needed. not sure if they sell by mail. I think the brand they carry is Specialized.

before you change another curbside flat though you should figure out what is going on. at one point I switched tires on a bike and used a higher pressure but I got flats cus I didn't realized I need to use thicker cloth rim tape at those pressures. good luck.

BTW: "you never regret buying quality"