ummmmm...that is a .090 sheet not 0.900 sheet.
If you have not yet purchased it, try:http://www.usplastic.com/catalog/item.aspx?itemid=23869&catid=705
24x48, 0.750 inch thick, 91 bucks. Not a great price, but not bad.
A Sherline will cut it, but be sure to google "Machining HDPE plastics" before you start doing much milling. Many plastics like sharp bits and high feed rates to reduce the amount of heat energy transferred to the material by the tool. Melted edges look bad and the stuff REALLY stinks when it catches on fire.