I have used foam core for stretching my watercolor paper for years with excellent results.  I tend to work with heavy washes and despise the buckling that happens to my works so I have multiple peices of gator board foam which I cut into standard sizes in order to use them with Imperial Sheets of watercolor paper.  You will find the following items to be very useful if you want to use this amazing material for your own paintings. 

ARTISTS PACK
INCLUDES THE FOLLOWING:
QTY:  1 SIZE:  12" X 16" FOR QUARTER SHEETS
QTY:  1 SIZE:  16" X 24" FOR HALF SHEETS
QTY:  1 SIZE:  24" X 32" FOR FULL SHEETS
THICKNESS:  1/2"
COLOR:  WHITE
Price:  $55.00 

FULL SHEET
PERFECT FOR FULL SHEET WATERCOLOR PAPER STRETCHING
SIZE:  24" X 32"
THICKNESS:  1/2"
COLOR:  WHITE
QTY:  1
Price:  $28.00

HALF SHEET
PERFECT FOR HALF SHEET WATERCOLOR PAPER STRETCHING
SIZE:  16" X 24"
THICKNESS:  1/2"
COLOR:  WHITE
QTY:  1
Price:  $20.00

QUARTER SHEET
PERFECT FOR QUARTER SHEET WATERCOLOR PAPER STRETCHING
SIZE:  12" X 16"
THICKNESS:  1/2"
COLOR:  WHITE
QTY:  1
Price:  $18.00