Little bamboo canes, or you may be able to find twigs off a tree or hedge, and strip any leaves off. Keep them apart, or they will try to entwine around each other (sexy things!)
When you do plant them out, leave the sticks in, I guess, as removing them could disturb the roots. I think at some stage pinching out the top bud will give more side growth, but you had better check that on a dedicated sweetpea site. Also, rotate them so they get light all round, and don't lean to one side.