Fabric manufacturers normally start taking orders for new fabrics about 6 weeks before each spring and fall buyers markets. Those fabrics are shown to shop owners on strikeoffs and even paper. Deliveries for those are usually about 6-12 months out, i.e. stuff we see in March usually start arriving in September and are staged monthly as they are printed, shipped to the USA, doubled and rolled on bolts here in the US and shipped out to shops.
So, shops are basically getting new fabrics almost every month of the year as they are prepared. Shops may also choose to space deliveries out, to smooth out cash flow.
The relationship that the pattern designer has with the fabric manufacturer or distributor usually dictates how soon that designer gets the fabric. In-house designers will usually get the digital images of the fabrics before the fabric is printed - so that sample quilt patterns using the fabric can be available as soon as the fabric is available.
Independent designers usually have to wait until much later in the game - often until after the fabric is available to the quilt shops before they are able to start designing quilts and/or sewing samples. Since quilting publications run on a 4-6 months-long publishing cycle, by the time patterns appear in the magazines, the stores have had the fabric for several months and are often sold out of some of the fabric used in those patterns.
In the last couple of years, fabric manufacturers and distributors have recognized this problem, and lately you're seeing many more patterns published in magazines from in-house pattern designers, i.e. those who have really early access to the fabrics.
Your best bet for getting all the fabrics used in a magazine pattern, especially one that is older, is to search online for the specific fabrics, and to go to online stores that specialize in those lines.
Also, don't ever be afraid to CALL an online store - except for the giant stores, most of them are run by quilters and are very happy to help you match and/or make suggestions on how to find fabrics even if they are sold out of them. It's called "customer service."
