They are only available when people are selling them. If you keep trying, you'll get one. If you give up, you won't get one until the market stabilizes a bit (or people start to log off and stop checking later in the evening, whichever happens first). It's up to you how badly you want these tokens.
Most players are smart enough -- if they actually sell WoW tokens -- to hold off for a while since the prices are going up, but many more don't realize that it's on the rise, or even have enough patience to care. Just keep trying. Or don't worry about it.