Shelters are very location dependent, unfortunately. If you're in a wealthier country / area there are likely some good ones around. Please get the kittens checked by a vet as soon as you can though, fleas can cause anaemia in kittens and kill them quickly. Even if you're rural, a vet who specialises in livestock likely knows of some local charities who can assist financially in providing care, if that's a concern. Kitten Lady's website is great for practical young kitten tips and product choices if you need to do some hands-on care yourself. Someone else already linked her YouTube videos which are also great, but I find the organisation a bit hard to navigate by comparison.

If the kittens look like they have pale gums/tongue or they are lethargic, or stop eating it's emergency vet time. Those are signs the fleas have caused anaemia. And it's important to keep them warm. 24 hour veterinaries exist in many cities. They frequently offer payment plans too.

They do need to see a vet soon though, even if they look healthy, so they can be checked and dewormed. Once they have been given the all clear, for good beginner-friendly long term care advice Dr. Uri Burstyn "Helpful Vancouver Vet is great. Also Jackson Galaxy for understanding cat behaviour is a fun watch for your family members who have fallen in love.

Please also consider getting them all desexed. There are programs globally working with vets to help get stray populations sterilised, which is recognised as the most humane approach to limiting cat populations. Cats also typically have longer lives when they're desexed, and are less prone to territorial behavioural issues like pissing all over your things.