Welcome to this excellent forum Michael. As David says, M-Machine is a good supplier as is MacMetals.
Try to avoid the unknown metals that well meaning people try to give you. They MAY be ok but they may also be difficult to work with.
Try to stick to a steel known as EN1A at least to start with. This is a free cutting steel that gives a good finish.
The EN series of steels were made obsolete many years ago but are still recognised by stockists and others because it is easier to remember than BS 970 230M07 which is the 'new' designation for it.
Aluminium can have similar problems as some aluminiums can be very soft and difficult to work with.
If you stick with one of the model engineers suppliers, as least to start with, you won't go far wrong.
All the best with your new hobby and keep asking questions on here.
Brian