fabric sources

I know this was discussed recently but I can't find it with the search function.

I'd appreciate it if someone would kindly recommend the best U.S. sources for tailored clothing fabric that will sell to retail customers, particularly lines such as Lesher, Smith, Porter & Harding and Harrison, based on both selection and low markup. I will be in London in June and would happily spend a day looking there as well.

Thank you,

Will