unregistered dealer

yes but he will have to pay tax on his behalf

Registered dealer can purchase from unregistered dealer but sec 9(4) says that recipient has to pay the tax on the purchase made from unregistered dealers.
Also note that Notification No. 8/2017 says that registered dealer need not pay any tax where such registered dealer makes purchases upto Rs. 5000 in a day from all unregistered dealers.