RDAP is standardized, so if I were you, I would just do it myself.
Step 1 - Getting the list of registry RDAP servers
Step 2 - For a given domain, extract the TLD, to identify which registry RDAP server to query
Step 3 - Send the request (use curl, python.requests, ..)
if the response code is 200, decode the json and get the data you need,
if it's 404 the domain is available and if it's something else, assume something went wrong