Believe it or not, but many of the array class methods convert the array to a Buffer, and then back to an Array.
See the Array.filter implementation in motoko-base - I would recommend just copying the Buffer filter logic/functionality from there. You can expand upon that to do the same for .find().
Buffer has .get() methods for retrieving indexes as well, so I would honestly only use an Array if you’re sure that your index-able list isn’t going to change in size - otherwise, Buffer all the way.