This post is actually about yarn substitution. That wonderful journey of discovery, that begins when you find the absolute perfect pattern - in a yarn that you just cannot find anywhere!! Sometimes the yarn is an exotic something-or-other that isn't imported into Australia: sometimes the yarn has been discontinued and you can't find a shop that has enough of it to make your dream garment.
In this scenario, the internet is your friend. Once upon a time, things that weren't imported into Australia were as hard to get hold of as bunyips. Nowadays, a quick internet search will normally turn up a little Midwest shop that stocks the exact brand of quiviut required for your pattern, and Ravelry has a page where crafters can sell or swap from their stash, so 'discontinued' doesn't necessarily mean 'gone forever'.
However, sometimes even the internet can't save you, and you need to find an acceptable substitute yarn. In that case, read on...
Step 1: Go to Yarndex and find out all you can about the original yarn. Yarndex has data for an amazing number of current and discontinued yarns, and you should be able to find the yardage, fibre, texture, gauge and recommended needle size for the majority of yarns here. (If you can't, Google the original yarn name - this information will be out there somewhere!).
Step 2: Google for yarns with a similar fibre content. This will find you a bunch of yarns that (at least in theory) will behave like the original yarn. You can substitute across fibres, but you may find that the garment does not turn out as intended - for example, if you substitute a cotton or silk yarn for wool, you will find that the garment becomes much stiffer and less elastic. A different fibre may also detract from a decorative stitch pattern: the example below relies on a bulky cotton yarn to give very sharp stitch definition. Wool is always somewhat fuzzier than cotton, which would make the lace less distinctive.
You now have a selection of yarns which are theoretically acceptable substitutes for the original. Go forth and yarn-shop!! However, you need to be prepared to knit test swatches and to accept the risk that you'll end up with a lemon. The guidelines above have mostly worked for me, but now and then you find that 'similar to' is a long way from 'the same as'. If that happens, I usually turn back to the internet to find a pattern for my new acquisition - and another substitute for my dream yarn.
* Suge Knight is notorious (among other things) for allegedly threatening to toss Vanilla Ice off a hotel balcony because Vanilla Ice had used some material from one of Suge's clients in his song "Ice Ice Baby".