You could try Sang Charoen Galvanize Co., Ltd, they probably will turn you down first, as you have only one item, but if you explain that you're not in a hurry. They maybe do it after they do a zinc electroplating for another customer. Also not forget, that they do no prep work, so you need to provide them with a bare metal product, any remains of paint, grease and rust and the job is not going to be of good quality.
Sang Charoen Galvanize Co., Ltd.
1/1 Moo 4 Tambon Khu cross A.
Lat Lum Kaeo,
PathumThani, Thailand
Tel. 0-2599-4115-8
Fax 0-2599-4197
http://www.sangchareongroup.com/