Since you haven't got a better answer, I can at least say that you are in the right place to start. I can't help directly but will keep your message and handle in case I run across something. 1889s are "relatively common," and your best (i.e. most likely to happen, NOT cheapest....) bet is a really beat parts gun for the forend iron.
A trigger guard can probably be adapted from another gun or a "stock" one from a gunsmith's supply store (Brownells? Galazan's?). Other members can probably suggest the most likely source for that, and since we have some real Remington experts here, I suggest that you do a search of the site for old 1889 posts and approach the most knowledgeable-sounding posters privately. Sorry to pass the buck like that, but an expert, I ain't.
Looking for an odd part like that is an intriguing and often frustrating project. I hope you are young or lucky, or both--it may take a while. But resurrecting an old double is a noble task and I very much like '89s, so keep at it. Not the most comfortable gun to shoot (my great grandfather's 10 has excessive drop), but graceful and tough at the same time, IMO. Good shooter with the right ammo. In those old posts you will find ammo advice, too.
Good luck with the project; let us know how it turns out!