I just wanted to let you know I was able to get one from Thrifty directly in Brisbane airport, even though I did not hire my car from them. The guy at Europcar had told me on the phone that they are friendly with one another (Thrifty/Europcar in the Brissy airport), and I simply asked (politely) at the Thrifty counter once I arrived in Brisbane.
There is a condition on about 20-30% of the vouchers that you "present your hire car keys (with Thrifty tag)" when using the voucher, so be aware that you may not get to use them all, but I did not have that problem at all. I saved a considerable amount at places we were planning to go to anyway - wish we could have used more but time prevented us!!!
Remember - More flies with honey. This last 2 weeks on hols, we got so many discounts and benefits simply by asking politely and by being genuinely grateful.