First things first; you must know the Outer Diameter (OD) of your well casing. Meaure this with a tape measure to the nearest 1/8 inch.   The OD of steel and PVC pipe is identical and there are two type of well caps;  The one piece caps are called "Well Conduit Caps" (WCC) while the two piece caps are called "Water Tight Caps (WTC)

If you want to purchase caps online, we recommend Suppy House.  They are one of our supplers and about half the price of other online suppliers offering identical quality products, they will often ship within hours of you placing your order.  Click on links below. 

4 inch wells with an OD of 4-1/2 inches uses 
4 Inch Well Conduit Cap WCC-A04
4 Inch Water Tight Cap WTLMC-A04

6 inch wells with an OD of 6-5/8 inches use
6 inch Well Conduit Cap WCC-A06
6 inch Water Tight Cap WTC-A06

6 inch Wells without an electric pump in them can use these simple caps
6 inch Plain Well Cap