where it will be available at cheaper price and what mm should i buy

You can buy 5 or 6 mm biofoam.
Cover one side of the foam completely with clear packing tape before using. It's pretty light, yet strong.
While building, make sure to keep the taped side towards the outside (For both fuselage and wing).
The tail surfaces can be made without taping the foam.
FliteTest uses the paper of the foam to make hinges, however since biofoam does not have paper, use tape to hinge your control surfaces.
YOu can search for tape hinge on google. You'll find lots of results.

Depron Available here :
http://www.vortex-rc.com/product-category/building-materials/depron-xps-foam/

Paper Laminated Foamboards available :
http://www.vortex-rc.com/product-category/building-materials/paper-laminated-foamboards/

Flite Test Speed build Laser Cut Kits Made in India also available :
http://www.vortex-rc.com/product-category/rc-plane-kits/airplanes-speed-build-kits/