Jim; you should be able to buy a pint can. Buy the time you locate and buy flattening compound, you might as well have just bought the smaller can of semi-gloss.
Try an automotive paint supply store...
http://www.automotivepaintandequipment.com/pdf/onechoice/OC-7_SLV4985_Universal Flattening Agent.pdf
Try an automotive paint supply store...
http://www.automotivepaintandequipment.com/pdf/onechoice/OC-7_SLV4985_Universal Flattening Agent.pdf