I often see a knitting pattern that I get excited about and when I look for the yarns required they're not available where I live, or a yarn shop might not have the color I want - or the specific yarn in the pattern is too expensive for me. When that happens - I go straight to yarnsub.com!
At Yarnsub.com I search for the yarn brand I'm looking for, wait a few seconds, until Yarnsub's database shows all the yarns that are similar and could be used as a substitute for the yarn I'm searching for. The search result will give a detailed description of the substitute yarns. The filter is another good way to find alternatives and find exactly what you are looking for.
This awesome online resource has helped me many times, especially when I didn't want to give up a pattern I was determined to use.
YarnSub is run by Wendy Peterson who is a life-long knitter with a fondness for colorwork and everything knitting. Over the years Wendy has often searched for a website(s) for yarn substitutions and couldn't find anything! When she decided to stay home to look after her two son's, Wendy and her partner David built an online database website and called it Yarnsub.