I have a 1998 555E and haven't had to buy a new pump yet. I have had very good luck ordering from Messicks from their online parts diagrams. I waiting on some gaskets from them that should arrive today.
My New Holland Dealer, George Baine in Tyler TX, has been very good at providing me advice and alternative sources for parts and repairs. It's almost like they don't want to do the work there. Their rebuild kits for hydraulic cylinders went crazy after Fiat bought out Case and New Holland. They added more and more seals to the kits and then made you buy a new piston. What used to cost $40 in parts went up to $100 or more, plus another hundred bucks for the piston. They told me where they have their cylinders rebuilt, and the guy buys just the seals needed, plus $100 labor to do all the work. Seals are usually $5
When one of my hydraulic pumps went out on my Case dozer, the dealer said that they do not rebuild them, or work on them, they will only replace the pump with a new one, which costs $25,000 for the pump. I found a hydraulic rebuild shop in Lone Star TX that rebuilt it for $3,000. I had to remove the pump from the dozer and bring it to them, then install it after they where done. It was a pain, but well worth the cost savings!!!