I figured you might know whether this is true or not from the HSUS:
Canned shoots offer hunters the unsporting opportunity to kill a variety of fenced-in exotic animals for a price. Many of these animals are originally purchased from zoos or game farms. They are familiar with humans and may even amble over to lick the hand of the shooter.
Last year, legislation (S.B. 784/A.B. 2612) to ban canned shoots passed the NY State Assembly but was stalled in the Senate. State legislators are back at the capital and we need to help push this bill into law this year. Even if you have taken action on this issue before, please take a moment now to call one more time and express your opposition to this unfair, inhumane practice.
Do zoos really sometimes sell animals for this?
I think canned hunts are a disgrace and should be banned. It takes a big man or woman to shoot an animal in an enclosed area that is used to humans and not afraid. I'd be embarrassed. 
Yes, "canned" hunts really happen all too frequently. Texas is probably the worst state for it to happen, but it happens in other states too. One of the deer farms I worked with in Ohio had "guaranteed" kills, where if a person paid enough money, they were guaranteed to have a trophy buck in a small enough of a fenced area only an idiot would miss shooting the deer---or the guides would shoot it at the same time as the "hunter", killing the deer humanely (one shot, one kill). If I remember right, a weekend for a hunt like this ran $8500 and up. These deer were "captive bred" but there is no way in hell you could pet them. They were essentially wild, just chute/gate trained.
No AZA accredited institution that I'm aware of would ever willing participate in such a canned hunt. That said, there are non AZA accredited zoos out there. I know for a fact there is one such zoo where a major figure in (I can't remember if he's the director or a curator or a collections manager, I just know he works there and has a key role with the animal collection) also operates a "big game/deer farm" in Texas. Also, no AZA accredited institution would willingly release nonpet trade exotics species outside of the zoo. I know several that will find homes for animals like goats, donkeys, chickens, peacocks, and other "domestic" species. This also includes some commonly kept "exotic species' like cockatiels or blue and gold macaws or buffalo. The AZA accredited zoos that I'm aware of MUST have a signed agreement of "non harm" for the animal to leave the zoo. This agreement is basically a document that says the adoptive individual will provide a good home for the animal, will not dispose of the animal (ie give it away) without the zoos permission, and will not use it for any illegal activities or breeding or food production or hunting. The zoos I know of that have placed animals also conduct site visits on those animals unannounced (I did one a while back for a pair of donkeys retired from a petting zoo). Also, typically those animals will preferentially be placed with zoo employees or zoo employee families, thus keeping them "in the family". If a zoo places an animal and doesn't abide by these rules, they can get into trouble with the AZA and USDA. All of that said, you have to consider the tigers, lions, and other dangerous exotics in the pet trade. Many of them were imported, but others are up for questioning. Again, I doubt it if an modern AZA accredited zoo would be involved voluntarily.
The sort of canned hunting is also rampant in Africa. There has been a big push for "biocanned hunting" where people pay money to "hunt" big game, but what they actually do is immobilize the animal under the supervision of a veterinarian, then once the animal is anesthetized, biological specimens (blood, feces, morphometric measurements) are collected, a cast of a foot print or horn (for rhinos) is made, and the animal is recovered from anesthesia. This gets off the "might hunters" because to use a dart rifle they have to get much, much closer to the animal--meaning its way more dangerous and its environmentally and ecologically sound, meaning that the animals are essentially unharmed.
I personally like the idea of doing that sort of a "hunt".
