I am not sure if there is any source for the uniforms of the thirties. Anyhow most of the armies at that time used to have more generilised and similar uniforms.
In general sailors looked like the WW2 era sailors and tropical white these days was in general use. Most of the armies of the world, were already clad the various khaki uniforms. A good start point would be the early WW2 uniforms, noting that minor powers still copied the uniforms of the great powers of the day, mostly on the grounds of their political and commercial relations.
Another point is that even today south american armies present similarities to the great armies of the time. One point of interest is that some of them used to have german-alike helmets etc. This is also the case for the spanish army, who had for example a helmet of spanish design which has the shape of the german one.
As a starting point and understanding that you need some more colourful units I suggest to use:
US marines in blue/sky blue uniforms, like the ones used at beginning of the century, pulp figures germans with filed insignias etc. For a minor inspiration see :
http://www.pulpfigures.com/new.php?custID=79167191651317031575it is the conversion bob murch provides.
for the british of the time see the interwar section of LAF and GWP
http://gwargamesp.18.forumer.com/index.php?act=idx choosing
http://gwargamesp.18.forumer.com/index.php?s=b8febfcbff8a7851170d8c2afa1895fb&showforum=4where you can find a host of information about the prewar uniforms.
SCW sites with uniforms also can give you some inspiration.
Info about South american armies is rather scarce and maybe some other members can give you more details on that.
Germans of the time it is easy to find. I would recommend those chaps by 'Pulp figures" like those you can find here:
http://www.pulpfigures.com/cat.php?range=Wierd%20Menace&catalog=PWM&custID=79167191651317031575Also, remember that many of the minor powers of the time were very conservative and their armies still had WW1 vintage equipment with minor modifications. Another good inspirational thread is:
http://leadadventureforum.com/index.php?topic=29611.0for making a south american army.