If you have a reciept you can order one from thermalright.
It should come in the box with the heatsink. Is there any particular reason you need another?
Did you sell the motherboard on and forget to remove it?
The only other thing you could do is DIY it with nuts & Bolts.
Best bet would be to email Thermalright and explain the situation. Might get one free, might have to pay for one. Never know unless you try.