To find a modeling agency you have to think about what you realistically can model. Most girls that want to model these days are not of fashion heights, but that doesn't mean you can't model. Focus on commercial print modeling agencies, casting directors, and working with agents that accept models of all heights for print work.
A girl that is not 5'7" or 5'9" or 5'10" can model. But shehas to have killer photos, quality photos, alot of ambition and drive, and she has to be honest with herself.
A girl that is under 5'7" can model for parts, -hands, feet, legs, ect, and there is a market for this. I would ask the agencies you are submitting to if they have a parts division. I would also make sure to have photos taken ahead of time of your parts, and taken in the style of an ad, look at shoe ads, and nail polish ads, try to replicate them in your shots. Also print modeling is something a girl of any height can do. I would also focus on headshots and beauty shots for this, focus on creating images that imply you can model. Create shots that show your energy, and a big help is looking at magazine ads and editorials. Focus on your strengths, and what you DO have to offer, don't dwell over and over on what you don't have. height. Focus on what you can actually model, be positive, and don't give up. If you want something you don't settle or give up. It isn't about an agency not accepting you, it is about finding the RIGHT agency for you.
You do have to do the work, create photos, and provide the marketing material to be a model. Yes you have to work first! Most agencies accept submissions, but you have to send snap shots or a compcard.
For more on how to get photos check out my Youtube videos: