Look at airsoft sites/shops and also search eBay and Wish for water pistols, the Japanese have been producing a lot of realistic looking water pistols as of late. Those are your best bets, although finding a Springfield specifically might be a bit tricky.
For reference, this is what IMFDB lists as Negan's Springfield 1911, it's a Springfield Custom Professional 1911-A1
I think that while there probably are plenty of airsoft, and other, replica 1911s out there, I'm not sure that anybody makes/sells this exact model. The model listed in the post above is likely the closest that you'll get. It should be easy enough to replace the grip scales but the scroll marks would be hard to replicate though and you'd have to cover the grips marks on the front of the slide, as well as adding them to front and back of the grip. Given that the one listed is metal, that makes changes a tad difficult, if you can find a plastic replica then it would be much easier to make the changes, esp. in terms of the scroll marks and the texture on the front of the slide and on the grips. You could putty over the slide grips and Dremmel in them on the grip.