Japanese uniforms changed significantly before the First World War and again in the 1930s. Russo-Japanese War figures really won't do.
The only dedicated range of troops in M.30 uniform I can think of are the Company B ones for Nomohan. I wouldn't recommend the sculpts and they are likely to be titchy next to Copplestone figures but your mileage may vary.
The aforementioned greatcoat mash up with Japanese helmets and arms is a good one. IIRC there was an article in WI or possibly the Warlord site by a chap who did just that. If you fancy non-greatcoated versions then Eureka did a range of Chinese in puttees many moons ago that with suitable head and equipment swaps will look make decent proxies. For the 1920s and arguably much of the following decade you really want flat caps rather than helmets, the helmet not becoming a standard item until the early 'thirties.
There may be some mileage in converting the Wargames Atlantic plastic WW1 Germans. Not tried it myself but I suspect you could get the arms to fit. The WG WW2 Japanese come with a wealth of personal equipment you can use for conversions.