How much current can the I/O sink or source?
This information is in the device data sheet, under "DC Characteristic - Over Recommended Operating Conditions." Check for the VOH and VOL rows, then find the Test Conditions column. This will show what the maximum guaranteed sink/source currents are for VOL and VOH (respectively).
The devices are capable of delivering/sinking more current; however, at values beyond those stated in the data sheet, the voltage and output timing delays can no longer be guaranteed.
IV curves can be found for the:
CoolRunner XPLA3 family in the device data sheets:
CoolRunner-II family in the device data sheets:
Note that the IV curves will not directly match up with the limits given in the data sheets. This is because the IV curves are for a typical device under typical operating conditions. The limits given in the data sheets are guaranteed for worst-case operating conditions and all process variations.
For other common CPLD questions, please see the CPLD FAQ: (Xilinx Answer 24167)