We have a decent selection for dosing/top off. The aqua house dosers are good quality and are only $99. If you want to go with a higher end doser you can go with the fauna marin pump. I personally use this on my system and couldn't be happier with it for my calcium, alkalinity, and magnesium dosing.
http://www.royalaquariums.ca/c/10/dosingtop-off