As far as I can tell, there are not many places (if any).
Aloha Rag will email you their inventory list and then you fax back your order. Do a search in this forum for Aloha Rag and you will find out more from the gals who have actually done this. By all accounts, they are great to deal with.
Maybe someone else will know of another place, like a boutique or something.
There is eBay, but you have to be REALLY careful.
Balenciaga does not allow the sale of their merchandise online. There used to be some sites that did so (e.g., Aloha Rag, CultStatus in Australia, Cricket Designer Wear in Liverpool, UK, etc.), but they all have since taken the pictures off of their sites and indicated that you must contact them re: available bags.
The only one that I've found that still displays/sells B-Bags is:
... but ... I'm not sure that anyone here on the forum has ever purchased from them (I find their prices to be rather high). I'm pretty sure that they sell other authentic merchandise (Prada, Gucci), but I can't vouch 100% as to the authenticity of the Balenciaga bags.
It's a store in Hawaii and due to licensing restrictions, they cannot actually
post pictures of Balenciagas online, but they do list the styles and colors
they have in stock and then you just need to contact them and they will
send you a purchase form that you need to fax back to them. 100%
authentic. I bought a Chloe bag from them last year.
You can also get them shipped to you via a store in Boston. You
can call them @ 781-237-7010. They have free shipping, I believe, and
you do not have to pay tax if you are from outside of Massachusetts
(A GREAT deal when talking about so much money). Tell them Eileen
referred you if you call. They can tell you the colors and styles they
have in stock and you can also request pictures via email.
Their website is GrettaStyle but they do not sell online. It's
just a boutique I used to work at: 100% authentic.