I'm right there with you - I loved the BBI holsters and would buy whole figs just to get them. Same with the Intoyz rig - I've still got one sitting on my workbench because I don't want to "waste it" on anything but the most worthy of customs
So in recent years, I've just decided to make my own. I take thin plastic (from a card bubble, transparency film, or something similar) and fold it into a flat tube, glue it together, and then cut it to fit whatever pistol I desire. It lacks high detail like the factory holsters, but it works perfectly for my purposes and they're far less bulky than the plastic/rubber used by the toy companies.
For my Selene custom (the first time I tried this), I used a little thicker material, so it kept wanting to "spring" open and required some re-gluing to get them to stay put:
https://fbcdn-sphotos-a.akamaihd.net/hp ... 1454_n.jpg
For Lara Croft, I used thinner plastic which made for VERY thin holsters -- I actually considered going back and bulking them up, but I used MU Comic Pack Bullseye pistols, which are pretty thin themselves, so it works fine. Here you can see my new version next to my old one (which uses the BBI holsters.)
https://fbcdn-sphotos-a.akamaihd.net/hp ... 9033_n.jpg
But yeah, I've never found anything even close to working as well as the BBI/Intoyz offerings. And hard-plastic holsters limit your options in many cases, so I just find it easier to make my own now. I'd recommend experimenting with different types of thin plastic until you find what works for you, if anything.
Hope that helps!