Since the cases are so close, use the same powders that are good for 38 loads. 231, Titegroup, Bullseye, AA#2, and many others will work fine. If you want something about halfway between 38 and 357, the midrange powders such as Universal, Unique, AA#5, are also good. Your best bet is to look in the manuals at the starting loads for 357, and see if that is mild enough. If it isn't, pick a powder listed for 357 AND 38, and reduce the starting 357 load towards the upper 38 load. Does that make sense? It sounded good in my head, but it's been a long day.