You can find Fluval online and in stores in most places, though prices vary hugely so shop around. It's rare to see the FX6 in stores but possible.
The SunSun's are online, Amazon is where I got mine, came very quickly (I think next day shipping was available). I got mine from TechnToy.com or something like that.
The Fluvals all come with some media, the SunSun usually doesn't. I just buy bulk media (I got it locally) and cut it with scissors, or there are some places that sell it pre-cut to fit. That's for mechanical filtration.
Parts are more readily available for Fluvial, both repair and consumables, though once you get into it the consumables for any filter are much cheaper to make than buy. Repair parts still might not be available locally, though, just more available online.
Regardless of brand, a failure of a key part leaves you without a filter until repaired. That may take days to get a part, which is one reason I recommended two filters. Two FX6's would be very expensive however, despite being (so I hear) great filters. Two 406's might be a better choice, but should you decide on one big filter, make sure you have an air pump and air stone (or equivalent) to provide aeration if the filter fails, to keep the fish alive. Cheap insurance.
Regardless, I recommend getting a bunch of Seachem Matrix for biological filtration. Fluval comes with ceramic rings I think, which are decent. Don't let someone sell you plastic balls, those are really for sumps and do not do well in canisters.
And I'm a fan of Purigen for chemical filtration if you need any, as opposed to carbon. It's rechargable, lasts longer, and most people report better success. Get one extra bag so you can rotate through recharging when you clean the filter (so preferably 3 for that size tank).
Finally despite being horrible looking, save the media from your existing filters to jump start the new, especially if the tank is rather empty of substrate (sand, etc.) or decorations. You want to make sure you have some good bacteria preserved across the change. In fact, I'd recommend taking two of the existing filters and cutting up the sponge media and putting into the new filter trays, run for 2 weeks at least with the third HOB
still going, then cut it up and add it also. You can rinse it in distilled or tank water (NOT tap water as it will kill the bacteria). The "gunk" on it that's thick and comes off easy is not too important, the bacteria is stuck pretty firmly to the sponges. You can remove it 1 or 2 changes down the line.
A tank with lots of surface area (that's not cleaned regularly), e.g. decorations, sand base, rocks, etc has a lot of the bacteria in the tank. One that is mostly just glass and slick plastic or similar surfaces may have most of the good bacteria in the filter. Think "is this surface easy to clean" - if so that's not where it lives. The good bacteria is NOT in the water.
Regardless, test the water for ammonia and nitrites regularly after changing the filter, and if you see any (meaning not enough bacteria survived the change), then accelerate your water changes until it is back under control and you only see nitrates. If you haven't read up on the nitrogen cycle, do so, but know you (almost certainly) have a well cycled tank, and are trying to preserve that during the swap.
As to problems with your water, if there are many (and you might want to start a different thread), but some quick comments:
- Dirty, ugly water with lots of crud may be bad, but may not necessarily be bad for the fish.
- Ammonia and nitrites from decaying food and other organics can be in very clean water, and can be deadly to fish.
So make sure you read about the nitrogen cycle, and keep the ammonia and nitrites down, THEN fix the other as you go. My guess is that you have algae problems of some sort. A filter may or may not help (the SunSun's ending in "B" have a UV
light that helps with "green water" algae for example), it is usually lighting (too much or for too long) or other nutrients you need to deal with, and/or get something in there that eats it, but figuring out a plan of attack requires identifying if it is algae, and what kinds.