I had intended to make my own antenna out of stretched sprue, but having just completed assembling Tamiya's T-55a, I'm not so sure now.

Do you recommend buying some soviet antenna or making them? If buying, which manufacturer?

In reality usual "whip" consisted/consists of four rods, each 1 m long, the first one the thickest, with partial rubber coating, the thinnest with small cast (flattened) ball at the tip. The length in use depends on the distance the comms should be kept - from 1 to full 4.
I suggest to buy metal one - did the same for my BMPs, bought aerial for BMP-3 incl antenna base (made by RB-Model, 2 pcs in the set)...