Ah, Irregular! Figures so uniformly awful they could be anything.
Those Orinoco figures don’t look the best, I must admit but it is the only dedicated range in 28mm. Easy enough to proxy with SCW figures. Empress’ Assault Guards would do for the Bolvians.
Funnily enough, of all the contemporay and near contemporary conflicts in the region, I’ve always thought the Chaco War, whilst the best known, to be the least interesting. The near contemporary, albeit much smaller, Leticia War between Peru and Colombia has much more interesting aspects including amphibious assaults, gunboats, seaplanes and the prospect of naval conflict. The 1941 Peru-Ecuador is even more interesting, a mini blitzkrieg, with parachute drops, dive bombers and mass tank attacks. For reasons that escape me the Chaco War is often, erroneously described as having the first use of tanks on the continent, despite the fact that they were used in combat much earlier in Brazil.
Adrian English is the go to author in English for all three wars, if only for reasons of accessability. Having read his books on Leticia and Ecuador, it’s obvious he relies heavily, perhaps exclusively, on secondary sources and has done little in the way of original research. That said they make useful primers. Another of his works that may prove useful is his Armed Forces of Latin America. It has information on armaments for each country from the early 20th C through to the early 1980s. He makes some odd statements, which do seem to belie an absence of knowledge of broader cultural and historical themes but none the less it’s a handy tome.