While the Magline Company is still in business, they no longer make trailers.
Most trailer parts, other than the frame, are off-the-shelf components. What specifically are you looking for?
I've completely rebuilt a 1972 Magline from parts ordered on-line through
www.easternmarine.com.
The axle was an odd diameter that I couldn't find matching bearings for. So if you're looking to replace bearings, you might need to replace the axle, hubs and bearings - like I did. The old bearings will have an identification number on the outer race that you can Google to find their size.
But for lights, tires, springs, hitch components and rollers, Eastern Marine is pretty convenient.