Ear mites infection again

Hello,
Yesterday I went to vet because my rabbit had a watery eye and the vet told that he got ear mites again and has an ear infection because of it (the vet gave him ivermectin shot and prescribed an ear drop). The worst thing is that I let him play with 4 female rabbit which the vet checked before that they didn't have mite because they were separated until I nurtured and spayed them and I brought them to the vet today and they all have been infected and will cost almost 1500$ and I can bearly afford it.

My questions are can I give them the ivermectin myself? And is it safe to do it?If so what dosage should I give them?
How can I make sure that they don't get it again? Is hay the cause ? or is because of a rat that I found in my house a two weeks ago?

Their ages ranges from 2-3.5 years and weight 4-5 kg and they all live indoor

thank you

why no one is replying last sunday the vet gave my rabbits their 2nd shot of ivermectin. can I give the next shot myself ? I still want to know how to prevent ear mites from happening again please help

Can you get Revolution/Sronghold(selamectin) antiparasitic? It's a topical spot on preparation used with cats and dogs, and it is usually considered safe to treat mites/fleas on rabbits. I think that would be much easier for you to do than to correctly calculate the dosage to give the ivermectin. Revolution usually only takes one topical treatment and you're done.

https://www.vgr1.com/revolution/
Medirabbit: ear mites

But if you're already treating them all and pretty much almost done with the treatments if the second shot has been given already, you could ask your vet to show you how to give the last injection. Or you can ask your vet if you can give it orally instead. I usually give it orally to rabbits.

Since we don't know your rabbits environment and living situation, it would be hard for any of us to be able to tell you where your rabbits could have gotten mites from. It could be from their hay, or it could be from pests(like the rats) in their environment, or somewhere else in their environment. It's impossible to know, unless you were to find the source of where the mites came from.