Harley-Davidson Bangkok, search for it, has Harley's for rental from 4500 THB per day for a Sportster 1200, up to a Electra Glide Ultra Limited for for 7000 THB per day.
In Phuket you also have several who rent Harley-Davidson bikes, probably the best known is Rider Cafe Phuket, cost 4000 bht per day.
I would love to rent a Harley-Davidson when come to Thailand next month, but I cannot find anybody who has Harley-Davidson bikes for rent.