Sadly, Astia 4x5" was discontinued about a year ago in the UK due to poor sales. It is now only available in the UK in 35mm and 120.
Our nearest alternative is Provia 4x5", which you can buy direct from our new film sales website, Fujilab: http://www.fujilab.co.uk/catalog/reversal-provia-100f-c-30_32_64.html
Otherwise, I would suggest that you try Japan Exposures - a Japan based photo distributor: http://www.japanexposures.com/shop/index.php?cPath=25&sort=2a&page=1
try b and h in the states if you are happy to import!
Let me know if you are getting some, might be a good idea to do a group buy!