The
Rickard banjo tuners are the ones you want. They will not slip, should be a direct replacement and if you tell him you are using them for harp guitar he will drill out the holes to fit the bigger strings.
You will need to decide if you are going to use "Bennett" tuning for your bass strings. This is named after Stephen Bennett. Gregg Miner can help you with that as well. Stephen has two DVDs that will get you up and running with songs to play and technique tips. You can contact him through his website.
If you really catch the bug, join us at The Harp Guitar Gathering, this year will be in Williamsburg Va Nov 8-10.