Props and costumes and scenery for Palm Sunday dramas from Oriental Trading
Search their site for "donkey" and you will find several useful things:
Donkey - cardboard cutout
A variety of donkey costumes, such as this one:
And you need palm trees. Search their site for "Palm Tree" and you find all sorts of trees, including:
Cardboard cutout
silk palm fronds
Need costumes? Oriental Trading has a entire page of Easter Pageant Costumes.
(I find Oriental Trading catalogs to be very inspiring!