If you have not done so yet take a look at the manual. There is a drawing of the parts in the back. go over the gun and order what you need from Ruger they are usually cheapest. To be clear, call and talk to them don't order online as the freight will be doubled.
If after you look at all the parts and still only need that one small part, I would post in the classified and someone may have a spare. In this case a used part should be fine. I may even have a spare.
Congrats on a great gun and welcome.
(The 89 was my first Ruger now I have a bunch of the P-series.)