Bonsai-En Has A Great Range Of Bonsai Wire In 500g Packs. Bonsai Wire Is Used To Shape Trees And Can Also Be Used For Tree Anchors And Guy Wire. Buy From Australia's Premium Online Bonsai Shop
4-8mm is generally used on trunks and thick branches
2-4mm is generally used for small to medium branches
1-2mm is generally used for detail wiring of much smaller fine branches.
This is just a rough sizing guide but here is a tip to help you choose Bonsai Wire for your trees
The wire should be around 1/3 the diameter of the trunk or branch. You can also pull around a 20mm length out of your roll and try to push the branch down with it, if the branch bends down the wire is strong enough for that branch, but if the wire bends and the branch stays in place you need a stronger gauge.