Women's jean sizes vary ridiculously from brand-to-brand, year-to-year and even within a brand. Also the shape of the fit varies a lot. The same can be said of men's jeans, actually. Your best bet is to go to the store and try them on, looking your self over in the mirror. Take a pocket hand mirror so you can see the fit from the rear, too. I can safely state for experience that you don't want to try to buy jeans online. Go to the store.

I'm replying to my own post here because I'm eating my words and revising my advice a bit. While I think it's generally ideal to try jeans on in the store, you don't always have that opportunity. And, of course, if you're presenting as male it can look a bit awkward in public. I have decided to experiment with online shopping of difficult fit things like jeans and shoes just to see if it's practical. I've researched a couple of dozen online stores and diluted them to the following list, which passed the following criteria:

* Well reputed dealer
* Good product selection (& some moderate prices)
* Sell both mens and womens clothes (etc.)
* Free returns

Free returns is the big catch with online stores and most online sellers do not offer them. But I figure in buying things like jeans and shoes I'm almost certain to need to return some things, so I'd rather not pay postage to do so.

After all was said and done I currently have only 5 winners, but they've got a lot of stuff:

The Winners:
Asos + + + +
Foot Locker + + + +
Gap/O.Nav/Ba + + + +
Macy's + + + +
Zappos + + + +

A few of Losers:
Penny's + + + x
Forever 21 + + + x
Oakley + + + x
Uniqlo + + + x
Urban Outfiters + x + +
Boohoo ? ? ? x
Am Apparel + x + ?

I even eliminated megaseller Amazon, though they do offer free returns on designated items. Returns must be dropped off at a UPS location, otherwise it costs $6 to have it picked up.

Anyway, I thought you folks might benefit from my research here. I'll keep you updated on my progress as I buy some things.

I've been wearing womens jeans since my teens, always found that they fitted and looked far better on me than mens jeans!
Way back then I had no choice but to go into the shop and ask to see if they would let me try them on and I would have to say that even back then most shops were fine, OK sometimes it was a bit of cloak and dagger sneaking me into a changing room or I had to take them to the mens changing rooms if they had a mens dept. 
As I always present as a man, so when I shop I tend to make sure I look as male as possible rugby shirt, jeans/skirt/leggings and some decent footwear, if that makes sense.
I find nowadays most shops don't care, a sale is a sale!

I wear woman's jeans all the time - no men jeans.  Nice fit,

For a year or two now I've been wearing women's stretch skinny jeans pretty exclusively and I'm sold on stretch jeans. True, they might not be true denims, but I love the flexibility of the more elastic material. I'm not a sucker for fancy labels, so Levi mid-rise skinnies do fine for me. I mostly wear the sort of faded looking blues. I did buy some theirs in black, but the black dye smelled up the place until I'd washed them several times. I won't buy any more Levis in black.

HELLO TO ALL THE SISTERS, I'M IVANA,
I ONLY USE WOMEN'S JEANS,

HELLO TO ALL THE SISTERS, I'M IVANA,
I ONLY USE WOMEN'S JEANS,

Please don't shout....thank you

Me too but that's because they are the only ones I can fit properly into, not through choice.

Yeah Ivana the shouting is sorta..... but decided I like your innocent enthusiasm anyway.
My bum in my jeans pic.

Edit as an afterthought:

How can it be that I just happen to have a picture of my bum in girly jeans laying around, what's the chance????

How many pictures can a person possibly have of themselves and what makes a person think, okay dressed to go to the supermarket, let's just take a picture of my bum first.

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My first pair were woman's Levis and they didn't fit that great. As I progressed, I started buying jeans at Macys on line and at Backstage in the store. OMG they fit great! The first pair were 12's and they make my butt look so hot! Then at Macy's Backstage I bought two pairs of Curves brand which is what i am wearing now and they fit great too. I wear these in Public and around the house and so far i have not been questioned. I think by body is becoming more female so they fit more naturally than that first pair of Woman's Levis.

I’m a tomboy who wore only boys clothes growing up. The conversion from men’s to women’s clothing is to add 20 to the female size to get the male size.

I’m a tomboy who wore only boys clothes growing up. The conversion from men’s to women’s clothing is to add 20 to the female size to get the male size.

Or subtract 20 from the male, for female size. I say that works for jeans or a skirt. That doesnt work for a bra top or dress. As i am 40chest that means i be getting a 20. I have 18