As this is a taught subject, the most heavily used titles on the RL may well be in the Law Reserve Collection, like the one below
Books with shelf marks beginning Internat 610 are on open shelf on Level 3, one floor above the entrance level
SOLO subject search
Use exact phrase Environmental law, International in subject index.
Texts on this topic are shelved at Internat 610, upstairs on Floor 3.
The following list is just a selection, starting with resource which require an Oxford SSO for remote acces, before moving on to some websites on the free web.