give www.muddbunnies.com a try. Lots of girls over there.
Really, I've never understood why people learning the sport think they need to stand flat footed on both sides over a bike frame. Realistically, when you stop, the bike tips over a little to one side and you put down one foot. That foot is usually on a rock or in a hole. I argue that the parking lot test with both feet flat on the ground should not apply when selecting a mountain bike fit.
I've ridden with a lot of women, and hear the standover problem all the time. Give a taller bike a chance.
Well, standover isn't so much of an issue for me now but it sure was when I was learning to ride. If you can't put your feet down in a parking lot you'll have a lot more difficulty putting even one foot down on uneven terrain where the ground may be lower than where the bike tires are or on steep terrain. Unfortunately, when learning and often walking tough sections or having to get back on in steep or tough sections, it can be really tough to get back on a bike with that extra standover. Also, it comes down to confidence. Knowing that touching the ground is an option may give one the confidence to ride a section. Once you can stay on the bike, touching the ground isn't much of an issue. I say a comfortable, happy beginner rider will excel faster than someone who feels like they're riding a high horse with no exit option.
As for claimed standover heights on websites, I've found that it means next to nothing. Check out every bike that you can in person and don't discount it as an option until you've confirmed the standover in person. With my 27"inseam (5'2"), I've been surprised by bikes that I thought wouldn't fit that do, and ones than I thought would and don't.
Also, keep in mind when checking out bikes, that they're probably oversprung for our size of person, so you'll get more sag and some useable standover when you get the right spring.