When to divide Welsh Onion?

[keepitgreen]

I got a Welsh onion clump from a garden centre on saturday and it looks like chives.

I was reading that it is a good idea to divide the clumps up so the bulbs can have more space to grow.

Can I divide it now when it looks like chives or should I wait until it starts bulbing at the bottom?

Also anyone know where I can buy Tree onions from?