Hi,
my name is Sabina, and I am co-founder of a multi-brand store. Are you running your store already? This is a very broad question to answer as I would need more information on the product you are selling. A good way to meet and address new partners ( boutiques ) is to attend fashion fairs with your collection of dresses. You should also do a research of who is your niche group and based on that look for suitable partners (boutiques) for you. Social media and partnerships with niche influencers are not only a way to build your brand but also a relevant reference for any boutique you want address. The best way to convince any store to list your dresses is to have a product that customers love :)