Ray McSavaney makes his own backgrounds using outdated 11x14 and 16x20 sheets of B&W paper. He exposes the paper to room light, then dips the paper into the developer. Through manual manipulation of the paper (letting the dev run over some parts of the paper more than other, twisting it, tilting it, etc) and a quick stop and fix, he makes some wonderful backgrounds.
He uses these backgrounds for his 4x5 flower photography. Stunning stuff.