Hey Steve welcome to the forums, I don't know for sure but you should be able to get the filter you want from any eheim stockist maybe if you put your location in your profile others maybe able to offer some local choices. I think those eheims are pretty dare have you considered an FX5, sure the eheim is a better filter but I don't think it is twice as good to reflect the price.. I have an FX5 and I am very impressed with it I have it on a heavily stocked 500L tank and only have to clean it out every 2-3months and water changes ever week or 2 I would see no reason to buy anything more than that.
I am not convinced that adding a canister will help with your problems, ammonia is dissolved in the water and should be converted by the good bacteria in your filter into nitrite and then into nitrate which can be removed by your water changes. If you are having ammonia problems then I would say your tank isn't fully cycled or the filtration isn't upto the task (flow too low, not enough bio media etc). How long has the tank been setup? Did you cycled it before adding fish? And what is in your sump and what is the flow rate?
Another option would be to buy a power head or a few bubble walls to keep the crud stirred up so it goes down the overflow into the sump.