It looks like Hot Topic has a huge sale going on for shirts, including a bunch of the pony ones they stock. Each is clocking in at just 10 bucks a pop, so it might be time to start grabbin them if the usual 20+ pricetag was a bit much.
They have a specific section for them, but a lot of the pony shirts are not present. Try this link, and dig around a bit more if there was a shirt you had your eye on previously.
This is not limited to online, so feel free to bombard your nearby hot topic for any they may be missing.