We have two companies of 'tree surgeons' who drop off a trailer load of bark chippings onto our allotments from time to time when they are working in our area. Our gates are kept locked so they each have a key and they usually give us a phone call to say when they'll be delivering. We have a designated area to compound chippings and they tip nearby. We found them by seeing them cutting down mature trees nearby, asked if we could have the chippings and they were delighted to get rid of them locally. We ask our plot holders who come and take the chippings for a donation of 50p for a large barrow load - the money raised going to the charities we support (this year its our local Hospice). We don't have chippings available in our compound all the time but when we do, we put a notice on our notice board saying they are available. This year, since February, we've had five loads and there will possibly be another load this week.
This works very well for us.