I have a year and half female Pitbull she is very healthy but we can not seem to get her to gain weight. She is very lean, muscle and very active. We can't afford the special prescription dog food. So was hoping some one here could give me some over the counter ideas. Thanks for all the help

At a year and a half she still isn't fully grown.
Yes, she is done growing in height but a lot of medium to large breeds won't start to gain weight and build muscle until closer to 2 years of age.

Is she skinny or just lacking muscle?
To test the correct weight of a dog there are a few things you can do.
First look straight down over her back.
The front half of her body where her ribs will be wide, then behind her ribs should dip inwards before coming back out again for the hips.

You should also be able to see the last 2 ribs moving under the skin when the dog is moving.

The third picture along here is perfect weight.

http://2.bp.blogspot.com/_83OotDXqde0/TAKUkaIoj1I/AAAAAAAAEu0/hx1l-dKi1_I/s1600/dog_weight_chart1.jpg