As announced on @banksygrossdomesticproduct on 11 December:
The T-shirts are only available for sale at an event in Bristol on 12 December. All of the proceeds go to four different homeless charities.
Banksy has collaborated with several NGOs over the years. One of them is, of course, the @lovewelcomes project, which among other things, made the coveted “Welcome Mat”, sold at the Gross Domestic Product.
“Love Welcomes is a creative social enterprise that helps refugee women begin to stitch their lives back together.” (https://lovewelcomes.org/pages/our-story)
Plenty of meaningful Christmas gifts at http://lovewelcomes.org.