Unbounded, 2-dimensional knapsack problem
Just the special case of (in your terms) 1-dimensional knapsack is NP-complete, and that is a special case of your 2-dimensional one (just take a strip of width one to be filled with strips of width one). It is very unlikely that an efficient algorithm exists for your problem.

Similar problems are handled e.g. by newspapers (need to fill pages with rectangles for ads, for instance), when cutting up sheets into custom sizes and similar applications. Search for those, there must be publications on approximation algorithms.