McMaster Carr sells solid chunks of both ABS and Acetal (Delrin) in sizes of up to 24"x24"x4".
However, I’ve had many prototypes of plastic parts CNC’ed in China from solid chunks of ABS, so I’m surprised your Chinese vendor can’t source something. While not the exact same as injection molded parts, its nice to test parts in their final production resin vs a 3D print material that only claims to be LIKE the final material.