I used B&D air retracts. Not sure if they are still in business. Mechanical retracts are tricky to install right so that the servo don`t buzz and drain your battery. You first got to have the time and patients to play with it until you get it right. Because you must get it right or else you will either kill your battery in flight and come crashing down or have many collapse gear landings, taxing or on take off runs.
I would go with air. They are a few places online that you could get a cheap set of main gears for under $125 or even $100. Now you may say air has its problems and it sure does. My B&Ds where the worse . Use spring air so that way if there is a leak the gears will come down. Now having said that , if you do set the mechanicals p right. You will have many trouble free flights and not have to worry about air leaks or remembering to refill the air tank before each flight. I have the H9 AT6 using mechanical and had close to 100 trouble free take off and landings. I cant remember where I got my retracts , might be from Tower Hobby. They work great. Just make sure they are the right one for your bird. I use a FMA S600 servo.