Sears won’t carry rebuild kits, nor will any representative of theirs know were to get one. Your best bet is an online supplier. Your local hydraulic supply store will likely have individual seals, but they may only carry pre made kits for the the jack brands they sell/service.
IMHO, the Sears 328 jack is one of the easiest designs to rebuild, and the seals used are quality. However, the only pic you showed … it looks completely rusted. I would advise disassembling the jack first to make sure that the internal components of the hydraulic unit are in good shape, and the frame of the jack hasn’t been bent or modified before spending the money for a rebuild kit. Yes, there are folks that have honed and polished internal cylinders and valve seats, but I think that extra effort should be reserved for something of sentimental value.