I found some of these listed on eBay. Here's the link below.
These are 8 3/4" in length, so they're 1 1/8" longer than the one shown above in 72-CLASSIC RIDE's post, but I think they may still work. The design of this strap looks to be very similar. They are sold in quantities of five.
.GM GMC Factory Correct Heater A/C Hose Wiring Harness Loom Tie Straps 5pcs C | eBay