I ordered mine from Ultimate Garage, an internet company in New Jersey. Prices are posted at this link:
http://www.ultimategarage.com/acjacks.html
My old Sears jack had blown the seal (like yours, apparently). I called Sears and they got a good chuckle out of my request for new seals for a 20-year old jack. Then I tried several hydraulic repair shops in town and they also expressed condolences. I looked at the aluminum racing jacks mentioned in earlier threads and decided that $900 was a bit much for my needs, so I wandered around the net and found these.
I had some difficulty due to the fact that Ultimate had just started carrying them and wasn't really familiar with the different models. The 2800 lb unit was listed, and during the few days that I was trying to decide whether to buy, the 4400 lb unit became available. Unfortunately, the literature on both AC Hydraulics' and Ultimate Garage's websites does not adequately differentiate between the physical dimensions of the two units, and so I ordered the 4400 lb unit, thinking it was the same physically. I also ordered the one of the cross-beam adapters and a rubber saddle pad; those arrived within five days of the order. The jack was shipped via UPS from a warehouse in Oregon; it took over three weeks to arrive, and when it did, the flimsy cardboard shipping box was destroyed, the jack was badly scratched and some small parts were missing. It also did not fit under the car. In fact, it didn't even fit under my 2000 Saturn LW300 station wagon. The local distributor told me that the 4400 lb unit is essentially designed for pickups and SUVs. I understand that one of the larger Corvette clubs had the same problem; they made a group order for a large number of the 4400 lb jacks and then found they wouldn't slide under the Vettes. Anyway, Ultimate Garage and I had several, shall I say,
warm discussions about the situation, and they finally got hold of the AC Hydraulics US manager, who coincidentally happened to be coming out to Mesa AZ the following week to set up a local distributorship. That individual brought out a replacement 2800 lb jack, and we were able to work out the swap locally through the distributor. Although not the most pleasurable shopping experience I've ever had, it all worked out in the end.
WRT whether the unit is rebuildable, I was told by both the dealer and the local distributor that they are.