You will probably have to order milk replacer online. And, no matter the brand, it's expensive.
While they promote their product, Progressive Nutrition has a couple good articles about raising an orphan. Even if you don' t buy their brand, I'd read the articles for the time frames for needing liquid milk, when you can switch to a milk pellet, how often to feed, etc. I'll get you a link.
http://www.prognutrition.com/pn/nutrition-information/what-to-feed-the-orphaned-foal/index.htm http://www.prognutrition.com/pn/nutrition-information/how-to-feed-the-orphaned-foal/index.htm
There are several different brands of milk replacer: Buckeye Mare's Milk plus, Foal Lac, Mare's Match and, I'm sure a few others.
Fortunately, I've not had an orphan foal, but have raised several orphan calves, it can take a bit of time to get them to take a bottle. I know with calves, they have to be on a bottle for 1-2 weeks due to their stomach design, but with foals being simple stomach animals, they can probably move to a pan or bucket fairly quickly. Pritchard nipples are often more acceptable to foals.
https://www.jefferspet.com/products/prichard-teat-nipple As, are some human baby bottle nipples. Lamb nipples would also be something to look at. Here's a whole set up of bottle and nipple (choose lamb nipple, and you'd probably only need the quart size bottle for a mini foal):
https://www.jefferspet.com/products/farm-babies-nursing-bottle
An article from TheHorse.com on feeding orphans (you might have to sign-up to read this one, but it's free to join):
https://thehorse.com/14390/feeding-the-orphan-foal/
Chewy dot com carries Buckeye Mare's Milk plus, so free shipping, but it's expensive:
https://www.chewy.com/buckeye-nutrition-mares-milk-plus/dp/170843 [Just be sure when comparing prices to other places to order from you consider the shipping costs (many places charge $30-40 for shipping on something this size).]
This place usually has $7.95 s/h for orders under #70:
https://www.fleetfarm.com/detail/land-o-lakes-mare-s-match-foal-milk-replacer/0000000231675?Ntt=milk
Horse dot com has two milk replacers (can often get shipping deals from them, but usually they are for under #15):
https://www.horse.com/item/buckeye-nutrition-mares-milk-plus/E016404/ https://www.horse.com/item/start-to-finish-mare-replacer/E014847/ I didn't link the multi-species milk replacer, as equine specific is best.
Good luck with your little one, sorry to hear mom didn't want to take care of her. If I can find the information, I'll see if I can get you information for a hormone protocol than can help a mare come around to deciding that baby isn't so bad (I came across it on FB, so not sure if it's available off FB).