One of the biggest challenges for new or growing brands when approaching multi brands (whether on or offline) is convincing the retailer of the following:
a) You can delivery the product as promised
b) It will sell and do so at an acceptable margin
c) That you can find a way to cover them if b) fails

If you are approaching them as an unknown and are getting significant push back, then your best bet is to ensure that they feel comfortable with the inventory risk they're taking. Here are 3 different ways in which that can be done:
a) guarantee their margin
b) sell on consignment
c) offer a return guarantee ("put it on wheels")

I generally only recommend c), as guaranteeing retail margin can become a drain on your own margin, while selling on consignment means you have to wait to get paid until the product is sold by the retailer. I therefore suggest to offer an upfront return agreement (either x%, or if you want to get really aggressive, take back anything that doesn't sell). The point is to cap their risk and make them feel comfortable about taking a shot with a new and unproven vendor. Make it clear upfront that this is for the starting season only to test the brand, so that they don't expect to maintain that kind of return agreement forever.

I can help with that! Do you have a catalog? Do you have a list of boutiques you’d like to get into? I’d love to review what you’ve got.

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 :)