Singapore Zoo has a population of about 16 proboscis monkeys now. Around 6 to 8 are on display at all times, the rest are kept off-exhibit. The Singapore Zoo's website is terrible, and I've been chiding them about it for the longest time.
Their diet at Singapore consists of about 20 different types of leaves collected from cultivated crops as well as gathered from wild trees in the surrounding forest.
Besides Singapore, a couple of zoos in Indonesia (Surabaya, Ragunan) and a zoo in Kota Kinabalu (as pointed out by Patrick) has them on display also.