I have read a lot of Mindfulness stuff over the years. My recommendation would be Jon Kabbat Zinn's books and CDs. The guided meditations CD especially is very calming and relaxing - I transferred them to an mp3 player so I could use them all the time.
In terms of free stuff, I found the free Destressify app is quite good. It has a small number of guided meditations. The Insight timer app used to be free and had hundreds of guided meditations, though I haven't used them in a long time. I tried Headspace but didn't like it, though others do.
YouTube has loads of stuff too. Just search guided meditations.
If you are looking for books to help with anxiety, depression etc. I can recommend:
Stop Thinking, Start Living by Richard Carlson. An excellent book on overcoming negative thinking. The Worry Cure by David Leahy is similar. Feeling Good: the new mood therapy by David Burns is a comprehensive CBT book.
Self Help for Your Nerves by Dr Claire Weekes is a great book. Some find it dated, but it is very comforting and has a useful method for coping with panic attacks.
You can get these on Amazon but try Abe Books first as most are available second hand for a couple of quid.
Like most things, mindfulness and CBT is not a quick fix, but if you are prepared to persevere and practise they can be very helpful.