Go to http://www.yarndex.com/home.cfm and find the yarn specified in the pattern. Once you know the characteristics of that yarn you can do a search for other yarns with the same characteristics (i.e., worsted weight, etc.)

Yes.....use a pattern that allows you to use the yarn and the needles YOU want. As I tried to explain when I spoke of one of the books I love, Sweater Workshop, you knit a swatch of whatever yarn you want and, using the formula given in the book, knit any one of the sweaters given in the book...this way you don't have to knit to someone else's guage, use any specific yarn or specific size needles...

You can use any yarn of the same gauge (# of stitches per inch) that is called for in the pattern. The pattern usually will tell you the gauge of the yarn they are using and you can also go to yarndex.com or many similar yarn sites that give the gauges with what needle size for many many yarns. Just google 'knitting yarn' and hundreds of sites will come up. Then look on the yarn packages where you wish to buy your yarn and find those with similar gauges, sometimes called tension. You can substitute any yarn of similar weight and gauge. No pattern expects you to use the exact yarn they are showing. But if you don't use one of a similar weight (lace, bulky, worsted, etc.) your garment will not be the correct size- a heavier weight will make a garment too big and a smaller weight will make your garment too small. You just need to find a yarn that is the same weight and gauge (# of stitches per inch) used in the pattern.

Your item may not look the same in more ways than fit if you substitute yarn. I, too, rarely use the yarn called for in a pattern... but I have had some problems when the yarn I used was more drapey than the called for yarn, or vice versa. If you use a yarn that knits up stiffly for a nighty... it may look like a triple XXX (and I don't mean size) Martian uniform when you are done... LOL

I recently substituted cotton-ease, which I really like, for the wool called for in a sweater - my sweater does not have the "presence" it would have in the wool... it's much more loungey looking - which is okay - but I wear it around the house instead of out. It looks like a sweater that has been well loved for years instead of one newly made.

Sometimes, if it is a popular pattern, other people have already made it... Google the pattern name and see if anyone has made it, posted a picture, and commented... you just might find it made in a yarn you like to work with.

There are just a couple more things which may be useful-- with all due respect, actually the patterns DO expect that you will use their yarn-- that's how they make their money! But to phrase it another way-- WE, the consumer, don't expect to use their yarn, all the time (with the price of so many specialty yarns these days, we'd go broke if we did.), and patty0315's advice is exactly right, along with hidiane's-- that is, check the guage requirements and go from there. The more experience you have, the better you will be able to judge the "drape" or "hand" or a yarn and if it will work with the pattern, as they are referred to in the industry. And here's a bit of a hint as well: softer yarns are spun more loosely, and stiffer yarns are spun more tightly. Also, the dying process will affect this (ever try "Strata", a self-striping acrylic? Stiff as a board!). So try looking at the yarn they suggest on the Internet and then eye to eye at Michaels, and see if you can get a sense of comparison. The other thing is that most people I know don't use exactly the needle size suggested in the pattern or on the skein of yarn, because they knit a little more tightly or loosely than the average calculated by the company or pattern maker, and need to go bigger or smaller in their needles to make the guage. But ultimately, I like hidiane's advice, which is to wear it around the house, if necessary and call it "well loved".:-)