Getting the right fabric is a problem many of us have experienced. I think it might be particularly hard on new quilters. A lot of people don't understand that fabric is ever-changing. New lines and colors come out several times every year, the typical brown of one time period isn't the same in the next time period. While sometimes you can indeed find more of something (especially if it is current) I treat fabric as a "one and done" if you don't get it right then chances are very good you won't find it again.
A lot of quilts in the magazines are designed specifically to go with particular lines of fabric, but you don't have to use those fabrics -- you can choose something close or something entirely different. I have one friend who has a very vivid sense of color, her selections when she took a class were always very different than the teacher or the other participants, but her quilts always looked like her quilts, while the other class participants projects basically just looked like the teacher's (which isn't necessarily a bad thing!).
Maybe the worst thing about fabric is when you have something in your head and then trying to find it... it may or may not exist. Fortunately there are now fabric printing options like Spoonflower.
Me, I mostly work in "scrap" quilts and am more challenged trying to keep myself to the same 3-6 fabrics for a single quilt. Scrap quilts can be very planned, with very small range of colors or fabric themes or whatever. The nice thing for me is if you throw enough fabric at it, each piece becomes less important and so you don't have to find the perfect fabric. The bad thing is instead of just going out and buying 6 pieces of fabric, I have to go through thousands of different pieces and I have to store those pieces so there is always at least two sides to any issue.
We all know what we like but it can be really hard to learn why you like what you like. For me I like to see differences in scale (the size of the pattern) between one piece and the next. I have vision issues and I like contrast which can be light/dark or color range. Some people like the watercolor effect and blurring. So when you are confronted with a "I like this but can't find that fabric" try to analyze what it is you like or at least what the original designer selected, so maybe you need a "large floral with a lot of cream background" or a "tiny little print of indigo on tan". And keep in the back of your mind that the designer probably could only use fabric from that line and might have really wanted something completely different!