For a 55g, I would have to go with a 20g Long sump. You can buy the aquarium at almost any retail pet outlet. For baffles, you can either get the glass cut at a glass company or you can go to home depot and buy window pane glass and a glass cutter. Use Aquarium safe silicone, or silicone that is NOT mold and mildew resistant or antimicrobial. Use masking tape to keep your baffles in place while you are siliconing them. The best baffle arrangement is an "under over under" style, where water goes under the first baffle, over the middle baffle and under the last baffle. This prevents microbubbles from entering the tank.
Of course, you will have to buy the proper pump and overflow for it too.