It's a lot cheaper to build one yourself! I'm sure that's what most people do. They are incredibly expensive through a retailer. Also, a 55 isn't that large. So, you would ideally aim for a tank at least held that size for a sump. I know people that run a 20 long for a sump on their 55 gallon. You can pick up a 20 long for 15 bucks and buy silicone and have a glass shop cut the prices of glass and you are good to go! I did this and the total was about 40$ ! Still running good.
Or get you a tank, then get one of these kits off ebay.... Seems like a pretty good deal. Lots better then 250.00 on a store bought sump. I also haven't found a lot of good reviews on the store bought sumps
I've had a few store bought sumps and they do make life alot easier. lol
You can get a sump off amazon for a 55g for less than $200.
You can also get a kit like posted above and use a petco dollar special tank. If you want to save a few bucks that is. me, I use both premades and homemades and they work pretty much the same way.
__________________ 180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18