Ok now that I have an aircraft for sale I see that airlines can offer leases with the option to buy.
This needs to be indicated in the lease agreements. If an airline signs a lease they should know before delivery if they will have the option to buy.
If you buy from a non-player aircraft broker you should always have the option to extend the lease or buy the aircraft. Here is a list of the brokers, as far as I can tell (did I list an airline, did I miss one?):
TAF Aviation Management
International Aircraft Sales Inc.
Eagle Aircraft Inc.
Allied Aircraft Investment Group
United Aircraft Brokers
Smith & Weston Aircraft Broker
Mile High Aviation
These are not that hard to figure out as they are selling various types of aircraft all the time, always give you the option to lease or buy, and always price the aircraft with not so round numbers.
When leasing from a player airline, you can see in the listing whether there is the option to buy, lease, or both (those little icons in the rightmost column). However, the lessor has the option to disallow lease extension at any time during the lease term. Therefore you are not guaranteed a lease extension and you should definitely not have a guaranteed option to buy during the lease term. If what you are looking for is an option to lease with a guaranteed option to buy, we need a new feature for the player airlines or you should just stick to aircraft brokers.