Here is where I got mine. http://ancofastenersales.com/our-products/raptor-plastic-staples?gclid=CMSJ3ZuMibwCFYhsfgodxWwA4w
Call for prices - I can't remember how much they cost, but a box will last forever if you are just using them on the CNC. I use the 15 gauge chisel point nails, and I have a box of 1/2", 3/4", 1", 1.25" and 1.5". I like the nail to go 1/4" into my spoil board, so I have different sizes depending on the thickness of my workpiece. I can hold a whole sheet down with 8 nails. If I'm nailing down hardwood more than 3/4" thick, I pre drill with a 1/16" bit so the nails go in easier and don't explode or split the wood.
If you use tabs, its nice to place a nail in each tab. They hold tight even when you cut them down, as long as the tab is 1/8" thick or more when the part is cut. This really secures the piece.
If I'm cutting out a large sheet of parts, I like to plan where I want the nails in Aspire. I draw a 1/4" circle where I want each nail, then I use a drill tool path to mark each hole with a 90 degree v-bit, .03" deep. This makes a small dot that is easy to see. I nail on the dots, then when I'm all finished I go back and drill each circle with a 1/4" up spiral end mill right flush with the spoil board (this is saved with the rest of my tool paths, so when I go to unload the parts the nails are already drilled out flush with the spoil board).