If you know someone with a lathe you could have one made, but Marlin is gone, and most parts have been snatched up by collectors. You might try the ususal suspects , but I think you will have to have something like that made for you.
Schlitz 45
A number of years ago, when I was deep into Marlins, I had to replace a few of these and I used these Bull's-eye sticks. Makes sense since they all have to be hand fitted. I do remember that these stick, use to be longer. ....
There are several people on the MarlinOwners forum that sell Marlin-contracted bullseyes, rather than reproductions, for a reasonable price.
(Like $3, last time I checked. But it has been a while, so don't quote me.)