For a good used gearbox, try ebay or keep scanning the "Parts" section of this forum until something turns up.

If the gearbox is just leaking, why not try fixing the leak? You can buy a re-seal kit for about $20 which contains every single seal for that gearbox. The gearbox is easily removed from the vehicle. There's even a rebuild write-up somewhere here (do a search). For a weekend of work and about 20 bucks, you could have nice power steering.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280