Years back,at least,Junction City was known as a place where you could buy nearly anything if you had the money.
Even stolen items from quarrys,oilfield,etc.,that were too "hot" to sell elsewhere could be sold or fenced in the area."Anything" that someone could steal from the Fort would be available around there,too.
I don't think it would be that much of a problem to get ahold of blasting caps,or anything else,if a person is willing to steal or pay blackmarket prices. :(
A blasting cap is certainly one of the more common types of detonators.
The news reports at the time said the muslims had traded a substantial amount of hashish for explosives stolen from a quarry. The explosives were of Spanish manufacture and also included a quantity of blasting caps.
Wasn't there even a report of them being stopped for some traffic offense with hundreds of kilos of explosives in the trunk of their stolen car? The car had not yet been reported stolen, so they got a ticket & went on their way.
The problem I see with Mayfield's involvement is that the Palestinians, the Greeks, the muslims in France, in Algeria and just about any place there are muslims, already know how to rig a cell phone as a trigger. Any moderately competent electronics tech could figure this out on his own in less than an hour given a cell phone he could sacrifice. Why anyone would go to the time, trouble and risk of involving a US muslim is a big question.
If Mayfield was involved, a significant possibility would be that someone wanted a hold on him. Blackmail him into participating on an attack in the US by threatening to reveal his involvement with the Spanish attack.