NI sells several PXI cards with your voltage specification. Go the PXI measurements page
here and browse through either the E series or Analog Output sections to find the product with the right price and specifications. Lower prices usually means lower accuracy and resolution and that's typically not what you want for a calibration source. Also, the magazine PXI Technology Review just published a buyers guide. You can go to their website at www.pxionline.com and check to see if there are links to other manufacturers.