Are you talking about a full cuncompressed BLU-RAY movie which can run up to 50GB in size or compressed / converted ?
A full BLU-RAY movie download of about 45 GB with a 5Meg DSL if you can get a download speed of about 500kbps will take you about 1 to 1.5 days to download. Not to mention your bandwidth usage that can get up to 300+ GB per month to maintain your share ratio. Make sure your ISP is unlimited and hope they don't throttle too much.
BitHQ, Demonoid, HDChina, CHDBits and the most sought after HDBits but unfortunatly these are all private trackers and you will need an invite. Best bet for smaller BD Rips would be to keep an eye out for Demonoid to open up it's registration to get started get your share ratio up and try to get an invite to one of the others.