Fully custom kitbash them or buy 3rd party.
The first Tyranid Prime I made was a kit bash using a Spawn of Cryptus Brood Lord as a base.
-Cut off the feet and added hooves.
-Trim off the top of the head and put the Prime head piece on it.
-Reapply the head spike from the spawn to the Prime head crest.
-Add tyranid extra carapace pieces along the back and on the upper legs
-Use monstrous Scytal and a bio cannon or Death Spitters.
-Use the hose from a monstrous biocannon as a tail and add the prime tail bit to the end of it.
My Swarmlord in a Forgeworld Hive tyrant with some modifications.
-Added Back vents
-Added a split end tail
-Added mandibles (made from rending claws)
-Added extra bits to the head crest.
-Bone Swords from Hydracast.