If you aren't too fussy, you could press some of the ACW figures in sackcoat into service as Argentines, at least in the summer kit. Ideally yo would want them with havelocks or you could make your own. If you were prepared to do a bit of carving of collars and repositioning of kit you could give thema wider use. Foundry's Mexicans are as close as you will get to Paraguayans apart from someone, whose name I forget has just produced a really awful looking set of......... Paraguayans.
Have patience and you might get a dedicayed range.