In the UK you can get via Skin & Me or Dermatica. Alternatively if you know anyone going to Italy, Greece or Spain or Asia it's available from any pharmacy there and it's incredibly cheap.
You start off really slow, you don't want to go straight for the strongest option. It took me over a year to go from 0.025 to 0.05%. My skin won't tolerate 0.1% and I see good results with my current strength so have chosen not to go up. You have to listen to what your skin can handle.
I use every second night. You also have to be very careful about using actives (like acids) when starting out. You also need to protect your skin barrier.
If you do decide to use tret, then I highly recommend Vanicream moisteriser and cleanser which are easily available in the UK and free of irritants. Or if you can access it then Illyoon ato barrier lotion and Aestura essence are both amazing and they are what I use noe, layered.
I also now use Dr Dennis Gross Daily Peel Pads twice a week (cut in half and put back into their packets so only really using one whole pad a week) and I have to say the results from them were instant and absolutely incredible.
I wouldn't use them when starting out with tret, but I also wouldn't be without them now. They have cured my adult acne and faded old scars and I have such a healthy glow.
Tret is great, but the peel pads give you that satisfaction of instant, incredible results.
Sunscreen every day, no matter what, is crucial. Your skin will be much more delicate and you will get serious sun damage if you don't protect it. Basically it causes the cells to turnover more quickly, so they are more vulnerable.