You can buy wool fleece in standard wool or merino. If you want to make your own stuff of course. Wool fleece is ca. £25/m roll, merino around £50/m roll, and you'd need ca. 1.8m-2m for a fleece jacket in XL.
If you want to buy it ready made, your options are probably limited and I am not aware of any.
I share your concerns on plastics, and have now moved to wool & Ventile instead. Have you considered just nice and cheap wool jumpers, a Loden jacket etc? Fleece is OK, but a jumper has many more uses and you can pick them up for sweeties in charity shops.