Loading gate, you mean?
The screw and gate are easily obtained.
They're available from all kinds of places.
Marlin
MidwayUSA
Brownell's
24hour Campfire (not recommended)
Numrich Gun Parts (used)
Western Gun Parts (used - not recommended)
And more...
The Marlin Model 444, Glenfield Model 30 / 30A, and a few other 'store-brand' rifles use the same parts.
Search terms that will help, if using google:
"Marlin 336 loading gate."
"Marlin 336 loading gate screw."
"Marlin 444 loading gate."
"Marlin 444 loading gate screw."
"Glenfield 30 loading gate."
Etcetera...