One thing I did was just look at the monthly threads on what is blooming. You can look at the August Blooms and September Blooms threads, and look for posters that are in your zone or a similar zone, and see what they are posting. That's what I did, and have stolen a lot of late ideas from Hazelcrestmikeb.
There are three ways to get late blooms. 1) buy daylilies that are registered VL, 2) buy daylilies that rebloom late, or 3) buy daylilies that are M or ML but are Bud Builders so they end up blooming into the late seasons.
Mike Huben has a lot of excellent rebloomers that can add blooms later as well. I have good luck with Magenta Kisses, Kanai Sensei, and But I Repear Myself in my zone. I like how late the VL's bloom for me that I got from Jim Murphy and Margo Reed at Woodhenge Gardens. I would definitely look through their site for VL daylilies. Some of their lates are bud builders, too. Cayenne and Fred's Red seem to build buds, and both bloom a long time, and quite late for me.
If you lean toward Tet daylilies, you should check out Scapehunters list. I love his L and VL Tet daylilies. He has some from Thomas that not everyone has, and they are really nice. Maybe tree mail him to see if he knows how to hook you up.