I have used the Jabscos on our previous boats. They are ok, but it seems that I recall the rebuild kits are expensive and a new head is not much more(not positive, so check it out).
I currenty use a Wilcox Crittenden. It has been reliable. The parts are easily obtaibable, including a nice schematic for them online. I found their online ordering very easy and friendly.
You can review their website here: http://www.wilcoxcrittenden.com/
I have also heard good things about the Lavac heads... but I have not owned one. You might consider looking at them too. This web site is here: http://www.lavac.com/
Good Luck in your search.