Lowes does not carry much in the way of quality acrylic material. IIRC most of what they have is polycarbonate, lexan or otherwise not the stuff you want to use.
You'll pay for good material, no way around it.
I would suggest "cast" acrylic, but "extruded" acrylic is also usable for sumps.
Try contacting altec on B street in boston, and Jfreeman in boston. Shipping will hurt on this, so local may be a good option.
I agree, usplastics.com is great as well.