No need to pony up a huge sum for an official replacement from Tektronix.
They can be ordered from Digi-Key for $4 (order code: 554043-1). Note that while M3.5 is the standard thread for GPIB screws, there is no standard for the thread on the connector. This particular one is UNC 6-32. If not replacing the above it may not match what you have. Check out the range here.