For other brands like Heavenly Solar, Hills Solar, to name a couple that are no longer produced finding the exact evacuated tubes, for a like for like replacement will be tricky and just about impossible unless you find a plumber that has a box of tubes tucked away. It is actually pretty easy to find replacement evacuated tubes for these systems but it will require some patience.
You will not be able to call up and ask for example Hills solar evacuated tubes as they are no longer produced under this brand. But what you can do is measure the outside diameter and length of your existing tubes. This can be done by measuring the tube outside diameter at the manifold on the collector. Once you have the tube outside diameter you need the length of the tube to make sure you can fit this into the clipping system on your collector. Some simple modifications may need to be carried out with the clipping of the new tube.
It is very important you do not throw the internals of your old tube out. I cant stress this enough as you will need to use these again in the new replacement tube. Once you have your new replacement evacuated tube sorted, simply slide it over the existing heat pipe and original aluminium internals. Then fix into place at the bottom of the collector.
Hills Solar evacuated tubes depending on the year produced should be easily replaced by a licensed tradesman with Apricus or Thermann tubes. Purchase the evacuated tube only, without heat rods as you will need to use your original heat rod and aluminium internals.
For other brands of solar hot water evacuated tubes once you have your evacuated measurements it is just simply finding what brands are available close to you and sourcing a suitable replacement.
Evacuated tubes can not really be posted or sent by freight with success unless you are trying to buy a box full.