A lot of times someone who wants to be a fashion designer will put together 20~100 shirts/pants/shoes/whatever and take them to local stores to try and see if the local store is willing to sell it. If people like it, and it sells out, the fashion designer will try to convince the retailer to put in an order for X more, and in the meantime they make X + 100 more, and take the other 100 to some other store as well.
So even if you have the brand, there may only be like 200 of that particular pair of shoes.
I've run into this problem before, I have a pair of white pants, that feel like their made out a plasticy, garbage bag-like material, I got them on eBay several years ago.
They got stained, and I went to try and find another pair. The brand was BIO Designs. Or maybe it was 310 Designs? Or something (it's not that I don't remember, it's that the font they used was ambiguous)??? Long story short, it took me 4~6 years or so of eBay/Google searches (once every couple months) to find another pair. eBay tends to be better because someone is always selling something and it changes more frequently. Google tends to be worse for anything like this (brand "out of business") because they don't show up on regular retail sites (if the brand is out of business, why would a retailer want something that didn't sell well the first time?).