I normally do a quick check, MDS , Tayna and Ebay. Occasionally Halfords have offers on Yuasa batteries.
I tend to check Tayna ebay site, sometimes can be cheaper and include free postage than buying direct from Tayna web site.
6 or 7 years is good for a Yuasa but then when you pay half the price and the other make lasts half the time, depends if you want to change more often.
One thing, I have contacted Optimate as my Z750 battery went down, the old optimates do take some life out of batteries if left connected but not turned on. Mine was connected but I turn the garage power off when I leave for safety.