Girls Communion Dresses are not so easy to be found in stores, especially with good quality, fabric and price. Nowadays most people are shopping clothing online so they don't lose time and have bigger variety. I think that's a good option for first communion dresses too, since you can find unique dresses with very good prices while shopping online.
I found an online store called Maria Communion with many beautiful and unique dresses, you can see above and bellow in the photos some of the white comminion dresses I liked the most. You can find more on their website and even choose custom measurements. I need to mention the fact that the quality is actually really good with fabrics like organza, satin and even beading, hand made flowers and zippers in the back! Let me know in the comments which dress you like more!