Actually you can wrap wire round a random chunk of steel. and there's probably some crafty sob that can figure out how to make it sound good too. LOL
But with regard to your question, while there are a myriad of grades of silicon steel, the big favorites that people like to wind with are M19 and M6. And if you want to get into some of the exotics, your other option is to use nickel alloy laminations. As a point of refernce I believe all the Fender trannies used plain jane M19 laminations and clearly they worked out just fine for them! M6 laminations with 413 interleaves and bandwidth spec's out to 250kHz or some other craziness like that is better left to the audiophool community. Just get your hands on some M19, paper or plastic cores, which really makes no real difference, and start experimenting and have fun with it.