I used a farret harness for a long time. chattiesmom makes silk step in harnesses and has a thread in the for sale section. There are also quite a few people on here that made harness vests that are really great and since they are custom made they will fit your baby. These are the seller's that I have used for harness vests, all three have very cute designs and the quality is amazing.
go.fetch™ The Chloe Bella Pet Boutique Tinkerbells Closet Dog Boutique: Designer Dog Clothes, Custom Dog Dresses, Sweaters, Carriers & More