Where can I get a vintage swimsuit set for babies like this one?

You can try online joints like Etsy.com or Amazon.com. There are hipster sellers/designers selling such vintage sets. If you are looking to buy them off the rack, I suggest Seafolly. They have both kids and adults swimwear so you might even find a matching set for you and your child to wear. Prices at Seafolly are on the high side (a similar looking vintage piece for little girls retails for 75SGD, pictured below) but reckon they will last you for a while. They have a flagship store at Wisma Atria and two other stores at Takashimaya and Great World City. Alternatively, you can order online at: http://www.seafolly.com.sg/shop/kids.html