Yes, a regiment of four-drawer filing cabinets standing side by side is impressive but few people have the funds or space for such an array. Some people are lucky to have a few two-drawer filing cabinets. Fortunately, by a stroke of bovine luck, filing folders fit perfectly into milk crates. Since original milk crates are rare or unavailable, the need has been filled by the plastics industry which makes colorful milk crates by the million. First, check your Sunday paper for auctions.
Auctions are very neglected sources of office supplies.
But do not wait for an office supply auction. Most auctioneers might advertise that the bulk of the auction involves something totally unrelated to genealogy. But, auctioneers also like to unload stuff from previous sales. On one occasion ten microfiche readers were bought at an auction which was advertising exercise and gym equipment. Plan everything ahead of time.