Here is a resource that I refer to frequently. It is the American Yarn Standards site which has a PDF file that lists the yarn weights, related gauges and knitting and crochet US sizes with conversions to European/Australian sizes. Bookmark the page and download the PDF, as you will use it all the time.
Now when I use 8ply yarn for patterns that specify worsted, I either use a larger hook/needles, or in the case of hats, I will add a row or 2 of increases and maybe a couple of extra straight rows. If I have the hat wearer handy, I will try it on as I go.
One of the problems I have with crocheting and knitting is getting the tension (gauge) right when using patterns from overseas websites. As I am way too lazy to make tension squares, I often find myself unpicking a hat that would fit a basketball, or chucking it in the back of the cupboard, not to be seen again until I do my semi-annual de-junking exercise!
I know you are saying that a tension square done now negates all that wasted effort later....but when you have to crochet, you just have to crochet!
I've been madly crocheting cotton market bags to sell. I've been having trouble sourcing good, cheap cotton yarn locally. I feel this is a huge gap in the Australian market, as most shops only sell crochet cotton, which is more for doilies, or expensive brands that are limited in colour/texture etc. This is one area where I wish I lived in the US...there are so many varieties over there and very cheap compared to here.
I did find a local site American Yarns which sells some of the more popular US cottons like Lion Brand, Bernat, Lily and Caron. Their prices seem pretty good, although they don't appear to carry the full range of colours. They use Australia Post rates which makes it a good idea to buy up, as they can pack into satchels.
If you are looking for high quality, basic Australian yarns, try Bendigo Woolen Mills. Their 4ply cotton is lovely to crochet, very smooth and no knots so far. They charge $12.00 per 200g ball which works out a little cheaper than other local brands.
I've also discovered some cotton (I think) yarn from Lincraft. They get bulk buys of Chinese yarn which they label All Sorts. The label is generic so you are never quite sure what you are getting but at $5 for 5 x 50g balls, you can always find a use for it. So far I've found some variegated 4ply (Sport Weight), some 8ply in fairly ordinary colours that will make good bags/washcloths and some really funky ribbon type yarn in a camouflage pattern that I used to make a cool market bag. (Pic to follow)
I haven't listed any of these things on the auction site that I use, Oztion but am planning to list some hopefully this weekend. In the meantime, if you want to see my other listings of mainly knitting patterns, look here.
Keep an eye out for my next post, where I will be discussing the merits of cheap vs expensive yarn, and sharing my thoughts on some of the most common Australian yarns.