Are the any substitutes for underlayment? The reason I'm asking is I have about 1 sq. meter of plywood to cover with underlayment, but I don't want to buy the whole roll. May be there's a way to cut the corners and substitute the underlayment with some vinyl or similar?
You don't say where you are located, but in the USA, there are different stores like thrift stores that have some construction materials, or the habitat for humanity ReStore, which carries only construction materials.
They have items that were donated, often leftovers from projects. You might be able to buy a partial roll of underlayment there if you have such a store nearby.