Hi, I get mine from an online pharmacy called Farmacia Andorra ( I think it's also called Farmacia Tristaina), the brand of tretinoin cream is called Retirides, it's a Spanish brand thats available in 0.025%, 0.05% and 0.1% strengths.
https://www.farmaciaenandorra.com/shop/retirides-0.5mg-tretinoina?search=retirides
I know it sounds a bit dodgy but I've been ordering from them for a few years with no problems. The first time I only ordered one tube, as I was worried it was a scam and I'd get nothing, but was desperate to buy some so I tried it (I'm not in the UK so can't get Skin & Me etc.).
Anyway it takes a few weeks for it to be delivered, but they should send you an email with a tracking link. Once when it got turned back at customs, they eventually refunded me the price of the creams (but not the postage unfortunately), but that's why I try not to order too many at the same time ( I think you're not supposed to send creams/ liquids through the post, it should be done by courier - but they send by post and it usually gets delivered fine).
The tubes cost about €20, and postage is about the same, so I usually order 3 or 4 together. One 30g tube usually lasts me about 2 - 3 months, applying it 4 or 5 nights a week as you only need a small amount. If you were using it twice a week or something it would last much longer.
I've read on here people also talking about a website called TretinoinWorld which sells a different brand, called Retin-A, you could do a search for posts and see if it's any good.
Or if you know somebody living in Spain, they can buy Retirides over the counter in pharmacies there.