Why cant i find a female zebra finch?

[apachetear]

i've been looking to add a pair of zebra finches to my other four as their squabbling is getting ridiculous and petty, i heard keeping them in 3 pairs would solve this.
So i went to my local pet shop to look for a male and female. They have plenty of males but seem to only have one or two females, which i always manage to miss out on!
Is their a logical reason as to why there are virtually no females?
I've had to go on preloved.com to find females, but noone ever gets back to me, which is probably as i cant travel to them due to health and transport issues.
I just want a female to make their cage complete!!
Sorry this is more of a rant than anything, but is there a reason why pet shops dont really have them?

[ac12]

I think it goes in spurts, for whatever reason.
Sometimes I've seen the local shop have way more hens than males, and other times the other way around.

I've also read that the mating hen can control the sex of the offspring. I think it was based on the availability of food. I don't remember which way it was, but I think it was:
- more food = more hens
- less food = more males
http://beheco.oxfordjournals.org/content/15/1/174.full
http://www.nfss.org/articles/article/Fo ... _Forum.pdf

However this excerpt goes in the other direction
- more food = more males
- less food = more hens
http://books.google.com/books?id=wz93GC ... ty&f=false

I have also read that the presence or absence of UV light can affect the sex ratio.
http://www.ladygouldianfinch.com/features_lighting.mgi
less UV = more males
more UV = balanced male/female ratio