These 5/8" Aluminum Head Push Pins are very sharp and are easy to push through the pelt and into the wooden stretcher.  They come in a pack of 100 pins.
 
Tip:  Although these pins are durable, you can get an even longer life out of them by not simply pulling directly out when they are embedded deeply in a wooden stretcher.  When removing pins from stretchers, push the pin back and forth slightly before pulling.