If you were in need of weaponry and supplies you certainly did. We can see that during Hannibal’s campaign whose soldiers would’ve looked more and more like Roman soldiers after each battle. The reason why the Romans went to Spain was to cut off his supply line and though unsuccessful initially (218 BC the Romans landed in Emporion, 211 BC the Scipio Brothers were killed [Martin Luik: der schwierige Weg zur Weltmacht, Mainz a.R. 2005]) they certainly did hinder the Carthaginians to properly supply Hannibal. However, he was successful in convincing several of Rome’s Italian allies to switch sides, revolt against the Romans and supply him, which would have included weapons and armors.

Now to Viriathus (since I researched quite a bit about him for my studies). In contrast to Hannibal, Viriathus was not cut off from his supply line since he fought a guerilla war in his home territory but you are right when saying the Romans went around plundering on the Iberian peninsular. By 143 BC Viriathus had already defeated 2 legions at which point Rome started sending consular armies again who were more successful in fighting and cornering him. At this point his soldiers would certainly have used the Roman weapons, armors and supplies they got their hands on. You have to keep in mind though that Spanish troops mostly consisted of light troops and light cavalry compared to the heavy foot soldiers and lack of proper cavalry of the Romans, and Viriathus would have plundered what he needed for his style of warfare. He probably had most use for the weaponry of the Velites (light spears and shields), the Romans swords (which were technically „spanish swords“ anyway), horses (though Roman horses were different from Spanish horses which were better for Spanish terrain), helmets, and of course, the weaponry of the Roman spanish allies.

The wars in Spain blead the Roman supplies and legions dry and they used a lot of spanish allied troops to bolster up their armies (something they did in every war: using local allies). Since several wars against tribes (you see that in Spain, Gaul, Germania and Britain) turned into guerilla wars, after a successful battle, the warriors were not only able to plunder roman weapons but those of their local allies, weapons they would be more familiar with.

Those are some sources I used when I researched the Spanish wars in my studies:

Keay, S.J.: Roman Spain, California 1988.

Luik, Martin: Der schwierige Weg zur Weltmacht. Roms Eroberung der iberischen Halbinsel 128 -19 v. Chr., Mainz am Rhein 2005.

Richardson, John: Spain, Africa, and Rome after Carthage, in: Hoyos, Dexter (Hg.): A Companion to the Punic Wars, Chichester 2011, S. 467-482.

Richardson, John S.: Hispaniae. Spain and the development of Roman imperialism, 218-82 BC, Cambridge 1986.

Schulten, Adolf: Geschichte von Numantia, München 1933.