Many thanks to rpengr for this highly useful post.
I wanted to add to it from my experience.
I have a 2004 H230 with a 5.7 GXI-F. It has a serpentine belt instead of v-belts. I don't know if that is original equipment. My pump part # is 21214599.
First, here are the options I found (pricing as of September 2014):
1. New pump from Volvo Penta dealer: $400 plus tax and shipping
2. New pump from on-line retailer: $280 on ebay with free shipping was the cheapest I found.
3. Used but rebuilt pump: about $200 on ebay
4. Get pump rebuilt: ebay seller 123gojoe offers to rebuild your pump for $160
5. Full rebuild kit: ebay seller 123gojoe also sells a complete rebuild kit (bearing, seal, circlip, impeller, o-ring, glycerin, and screws) for about $80. You do the work.
6. Buy parts: if you're willing to put in some time you can probably save a few dollars more sourcing the parts elsewhere, but the total savings will be small so either your time must be worth very little, or you'd better enjoy the process.
7. Just the seal: the seal is pretty easy to come by (google the part number: 827247) My local Volvo Penta dealer had it in stock (but still told me there are no repair parts for the pump).
For my pump, it turned out my bearing was in good shape, so I'm just going to replace the seal.
My pump was a little different that rpengr's. Instead of the bearing being held in by peening, there is a large circlip. This is a view from the backside, after the pump was pulled off the pulley/shaft:

Because of this, both the bearing and housing must be pulled off the shaft together, and then the circlip can be removed, and the bearing driven out. While I didn't remove mine, I did come across a nice trick for doing it. The problem is there isn't an easy place to drive against to move the bearing out. The trick is to use L-shaped allen wrenches, placed through the shaft hole against the bearing, arranged back-to-back, and then driven lightly with a hammer a couple of taps; rotate about 90 degrees; tap tap; repeat. A picture should make it clear:

I tried it, and it worked great. I was afraid the wrenches would buck and twist all over, but they were pretty stable.
I hope this helps and adds to the collective wisdom.