For such a small amount I'd just buy a 300×300 sheet which is what most of our suppliers seem to do and cut them with a saw and tidy up by hand or mill if you have one.
If you do buy it in also consider how you want it cut, eg will a sheared edge be acceptable or would lazer of waterjet be better. And as mentioned above what tollerance, maybe allow a bit for final finishing.
And if it is a bogie make sure your cross piece length allows for the fact you will be getting metric material and your imperial lengths may be a fraction short.
If you can put the pieces into a .DXF file then you can just e-mail it to get a quote which makes it easier for the supplier as most have computerised machines these days.
J