It is not mandatory to change both ends together. However if you install new shocks, they will lift the rear end a little and transfer more weight onto the forks. If you lift your bike up so the forks are fully extended, then compare them with when it is back on the ground, with you sitting on it, you will find you have very little travel left.
So it is probably better to do your forks first, so you have more travel to deal with any poor roads. You can improve them by simply adding spacers on top of the springs, to reduce sag.
As I said earlier, it is easy to change rear shocks. Hagon
is a British firm and you can probably order direct from them. They can also supply fork springs.
Take care with using the word 'progressive'. There is a US company named Progressive Suspension Inc, who are a HDF sponsor and they also may be able to supply both shocks and springs. Some springs for both front and rear suspension are described as 'progressive-rate', often abbreviated to 'progressive' and here on HDF the suspension company is often called that too!
If you do want to buy, contact your chosen firm by email first. Also make a point of ordering them with springs to match your weight. In addition to the purchase price there are likely to be quite high shipping costs, plus any import taxes your Government charges.
Take some time to read through suspension threads here on HDF, so you get familiar with what can be done. Obviously you won't be interested in lowering your bike!