If all you are doing is building baffles, then the 1/4" panels from Lowes/HD will work fine.
But if you are building an entire sump, you probably want to go to a plastics supplier that can get you cast acrylic. The stuff sold at HD/Lowes is extruded. It tends to yellow and swell up over time. I built a sump once with this stuff and it lasted about 18 months before seams started busting due to the swelling.
If the sump isn't large, you can order small cut pieces from US Plastics or any number of other suppliers. If it's a big sump, I'd look for a local plastics company, or just eat the freight charge and get a 4x8' sheet.