I couldn't find plexisheet near my craft stores so I end up buying a plastic bendable chopping board from walmart which comes in 4 different colors. This can be cut in any different sizes. Do u think that would work for sharp edges on cake? Please help?

Since you've already bought them, they will probably work fine. I would have gone to the hardware store and bought a wide taping (drywall) knife.

I went to home depot & didn't find one :(

Is this for smoothing icing?  If so, have you tried a bench scraper?  If not, what's a plexisheet?
 

You're using the plexisheet for the upside down method, right? I went to home depot and bought a nice sized inexpensive sheet of glass to try it out. Might be worth looking into next time...

Fat Girl Cakes sells taller bench scrapers, safe for food use. Just google, you'll find her.

I bought plexiglass thick plastic sheet from home depot for upside down technique.. you are right..!!  about $5.  But Here I am asking for plexi thin plastic which use to smooth the fondant & make nice edges on fondant cake.

Go to the paint dept in home depot and get a paint edger.  It is about 12" long and is about 4" wide.  I use mine for smoothing icing.  While you are in the paint dept. pick up the longest sponge roller on a frame that they have.  After your buttercream crusts you roll this on it to smooth it out.  I swear by that thing.

Thank you so much :)