Automatic postage stamp affixer machines such as the Accufast XL and the Postmatic stamp affixer require machine grade stamps from a roll.
The rolls of stamps need to have the following characteristics:
A gap between each stamp
A 3 inch core diameter
The USPS sells these in rolls of 3,000 and rolls of 10,000. Both the Postmatic and the Accufast XL are compatible with both sizes of rolls. These can be purchased on the USPS website and also at many local post offices.