Does anyone know if it is possible to buy a powder or something that will stain skin if someone touches it? have tried googling and not found anything.

You could try asking your local Crime Prevention Officer.  They could be helpful with other suggestions too.

Google 'Smartwater'.

Smart water seems to mark property so you can prove ownership. I want something that will stain skin when someone has sticky fingers where they shouldn't. Thanks anyway.

I have no idea if you can get smart water but it does mark skin ad clothing too for a period of time.

Google turned up this and this ...