I get 1 x 1 x 6 inch blanks and they are big enough. Yeah, there's lots of 5 inch blanks out there and I suspect that's what you have near your house? Yep, too small.
The 6 inch blanks I got here Monster 1 x 6 Inch Acrylic Blanks, Acrylic Pen Blanks, Acrylics I think this one is "fishbone maroon". There's other places have the big ones too.