For imports and prices I have recently been using dvdpacific
http://www.dvdpacific.com/Default.asp. They are usually cheaper than most other places but you have to be prepared to wait at leat 1 week if you choose their cheapest shipping option (USPS surface mail). Recently, cars, blade runner and several other discs have all taken 7-9 days to reach me (from date of posting) which is quicker than the time frame they state. They are also very good for service and I've never had any reason to complain. I can't comment on customs charges from them as I have never purchased anything over £18 from them.
All HD DVDs are region free so you don't need to worry about them. For blu-rays check this site:
http://bluray.lindsite.dk/
Movietyme are also good for imports and delivery is included in the price
http://www.movietyme.com/catalog/
with movietyme shipping times can vary from 1 day to several weeks but in my experience the discs always get to me (and they are also customs friendly). Movietyme also compile their own blu-ray region free section (which they test on their own UK PS3)
http://www.movietyme.com/catalog/default.php?cPath=355
If you are searching within the UK then this site will give you the best price for the films you are looking for:
http://www.find-dvd.co.uk/
More often the not the cheapest places will be
http://www.sendit.com/,
http://dvd.co.uk/,
http://www.blahdvd.com/blah/Home.aspx or foxy (I however stay away from foxy as people report them as being rubbish).