home made (seperately purchased) wireless transmitters would probably be best, since you have more selection and control over the range.
i'd say, get a wireless transmitter that can do the distance.. and then get one of them portable headphone amplifiers to power the headphones, because the wireless receiver probably wont have the power to run the headphones.
you will save money by not asking the wireless transmitter to amplify the sound for the headphones.
and when you spend the money you saved, that should be for increasing the distance.
the portable headphone amplifiers are common on ebay.
some of them come in a can of altoids.
some of them are in plastic/metal housings.
the point is, these things use a battery inside so it is portable power.
you could just toss the amp in your pocket and put the wire under your shirt to keep the cord out of the way.
if that doesnt seem like a practical choice.. then the alternative would be to use a repeater (or extender) to transfer the audio from the radio to the wireless transmitter.. and from that wireless transmitter to ANOTHER wireless transmitter that comes with the wireless headphones.
you will always have a piece of equipment outside, maybe you forget it out there or it becomes a hassle to setup and turn on when you want it.
doing it piece by piece should give you more power and control to use only one wireless transmitter.
again, that means putting together a wireless system piece by piece yourself.
most wireless headphones are 30ft - 50ft without a wall in the way.
that is kinda like wanting to buy a new car and all you are being offered is riding lawn mowers.
you'll get to choose the sound quality of the headphones.. and this should be a real reason.
you will also get to choose the quality of the headphone amplifier.. another real reason (because you dont need high quality headphones with an amp that makes 'em sound like they arent high quality anymore)
and you will also get to choose the wireless range.. another perfect reason.
it is usually this way or buy an mp3 player or satellite radio player and pay the subscription fee.
i dont really want to steer you away from your desire of wireless headphones.
you might have to put the wireless transmitter that came with the headphones on the ground in the middle of the garden .. connected to another wireless transmitter.
i have put a cord under my shirt with a player in my pocket.. it isnt totally wireless, but you should be able to forget about the cord (unless the headphones fall off your head).
a piece of tape on the backside of your shirt could hold extra wire out of the collar to keep the weight of the cord from being yanked on by gravity.
you could put a piece of glue onto the cord, and use the glue to act as a stop with a safety pin.. you wouldnt be wasting tape then, but putting holes in the shirt.
most of the time, they offer these products and they are fun.. but they are made for a minimal work effort.
if you want to go larger, you need to build it piece by piece yourself.
and then sometimes they make it difficult to find the parts needed.
unsettling for me when somebody comes along and asks for more quality than generally available.