Now that Twitter has totally killed their V1 API there is no official way to get Twitter feeds via RSS. Which is a real shame as RSS is a well accepted Open Standard for this type of information :-(
The Twitter REST API v1 is no longer active. Please migrate to API v1.1. https://dev.twitter.com/docs/api/1.1/overview.
SHAME ON YOU TWITTER !
Currently there seem to be very few third party sites providing RSS services and it appears unlikely many will as Twitter will just kill them with changes to their API and/or terms & conditions as soon as they gain traction.
Here is one I found to get a Users Timeline (which is not all that useful).
However Hashtag or Search to RSS services are harder to find and will need a lot more work to implement
Please use the comment section if you are aware of others !