A foam hot tub cover is a popular option for a tub, especially if it is covered in vinyl. This type of cover can be used indoors or out. Here are a few places you can buy the materials to make the cover.
You can purchase foam that is best used around wet surfaces from a good upholstery supply store or outlet.
Hot Tub Suppliers
Some hot tub suppliers and distributors may sell the type of dense foam that is used for this purpose. This type of foam is also used for upholstering cushions and seats in boats. It is not prone to mildew or molds.
If you purchase foam from a general purpose fabric store, be sure to tell them the intended use so they can sell you the proper type.