You can access the internalId of list items with GET /lookups/{fieldname}, replacing {fieldname} with the name of a picklist.
The response for GET /lookups/currency looks like this:
[
{
"internalId": "1",
"name": "USA"
},
{
"internalId": "2",
"name": "British pound"
},
{
"internalId": "3",
"name": "Canadian dollar"
},
{
"internalId": "4",
"name": "Euro"
}
]
