Walker jacks have changed ownership and name about 4 times over the last 20 years. Currently they're sold under the Hein-Warner name. You might contact them to see if they still supply repair kits for the Walker jacks. There are some seals in those jacks that you WILL NOT find in any sort of regular industrial supply house.
Another option is to take the jack apart and take the seal parts to a hydraulic seal shop if there's one in your area.
If there's no one local, you could likely match the parts up online at http://www.herculeshydraulics.com
Based on work I've seen in the past, I'd have to have some good recommendations from prior customers before I'd take a jack to the typical hydraulic jack repair shop. I own a machine shop and do some hydraulic repair work, and the worst messes I've ever worked on were ones where some jack repair yokel had "fixed it'. I've seen pieces of PVC pipe used in place of plastic piston bearings, cut up wheel cylinder cups used where a U-cup seal should've been, and more other examples like that than I've got time to list. Not saying they're all bad, but I'd want a reference from someone who'd had work done that has held up for at least several months before I'd turn one of 'em loose on anything of mine.