They are listed as of 31st May as being on pre-order from:
- https://shop.pimoroni.com/collections/micro-bit
- https://thepihut.com/collections/new-products/products/micro-bit
- https://www.kitronik.co.uk/bbc-micro-bit-accessories/bbc-micro-bit.html
- https://www.rapidonline.com/563128
I would recommend any one of these vendors as I regularly order from them all.