The contract contains ether. Can I buy the gas within the contract code?
Yes and no.
The gas has to be paid for, up front, by the original sender. There's no way to add more gas inside a transaction.
However, it's possible for a contract to reimburse the sender. It's also possible for the transaction to have a gasPrice of 0, and then the contract reimburses the miner via sending to
msg.coinbase. In the latter case, a miner would have to be specifically programmed to accept that kind of transaction.