Always best to get a doctors prescription and have it compounded by a local pharmacy but if that is not possible is then there are other options.
This site sells it without prescription - https://www.riverpharmacy.ca/drug/naltrexone-nodict
and even cheaper here - https://www.alldaychemist.com/415-naltima-50mg-.html
Can mix 1 x 50mg pill in 50ml of water and take 4.5ml daily.
Buying prescription drugs over the internet is a pretty risky business and not recommended but the stuff these guys sell appears to be genuine. I think gbash buys LDN online so he may know of other sites.