No distinction is made in the regulation between “naked” or “covered” short sales. However, the practice of “naked” short selling, while not specifically enumerated or proscribed as such, may violate other provisions of securities legislation or self-regulatory organization rules where the transaction fails to settle. Specifically, section 126.1 of the Securities Act prohibits activities that result or contribute “to a misleading appearance of trading activity in, or an artificial price for, a security or derivative of a security” or that perpetrate a fraud on any person or company. Part 3 of National Instrument 23-101 Trading Rules contains similar prohibitions against manipulation and fraud, although a person or company that complies with similar requirements established by a recognized exchange, quotation and trade reporting system or regulation services provider is exempt from their application. Under section 127(1) of the Securities Act, the OSC also has a “public interest jurisdiction” to make a wide range of orders that, in its opinion, are in the public interest in light of the purposes of the Securities Act (notwithstanding that the subject activity is not specifically proscribed by legislation). The TSX Rule Book also imposes certain obligations on its “participating organizations” in connection with trades that fail to settle (see, for example, Rule 5-301 Buy-Ins).
In other words, shares must be located by the broker before they can be sold short.
A share may not be locatable because there are none available in the broker's inventory, that it cannot lend more than what it has on the books for trade. A share may not be available because the interest rate that brokers are charging to borrow the share is considered too high by that broker, usually if it doesn't pass on borrowing costs to the customer. There could be other reasons as well.
If one broker doesn't have inventory, another might. I recommend checking in on IB's list. If they can't get it, my guess would be that no one can since IB passes on the cost to finance short sales.