Bespoke, made to measure harness can be expensive and you need to know where to measure to achieve an exact fit.
For the 'average' 10.2 -11.00 hh a shetland pony harness often gives a good basis from which to make any changes. Pony size harness will generally be suitable for donkeys 12hh - 13hh.
The Zilco range of harness provides a readily available harness with the benefit of having all pieces interchangeable for another size, ideal when creating a harness for a donkey. Their donkey harness is meant for donkeys 12-13hh and so is not suitable for the majority of donkeys in the UK. However, for donkeys less than 10.2, Zilco now do the SL mini harness set which is in proportion for smaller animals.
As an example of how a donkey harness might be made up, the shetland sized tedex harness from Zilco provides a good starter harness for a standard donkey but will need a pony size crupper strap, probably pony size traces, breeching strap and maybe reins and for the bridle probably a cob size headpiece, cob size browband, pony size cheek pieces, pony or cob noseband and pony or cob throatlash!
Most suppliers of Zilco harness will mix and match the individual pieces of the harness. Trudy Affleck is happy to help with harness ordering or fitting - please contact for more details.