Most Home Depot's and some Lowes carry 4x8 sheets of melamine.

In most larger towns, there will also be a professional
cabinet builder supply that will carry the larger sheets.

Melamine has fell from favor in cabinets and has become
harder to find.

Be aware that a 4x8 sheet of melamine is VERY heavy and
very slick and difficult to deal with.

It will require a high count(80+ tooth) blade for chip
free cuts.

wrote:
Is it possible to purchase melamine sheets like the kind used for
shelving in widths greater than 11 7/8"? I asked at my local HD and
Lowes, and building supply companies with no success, both bigbox
places said it was not manufactured any larger than shelving width.

Cheers