As you probably have discovered, 9mm bullets are usually around .356 in diameter.
Many molds cast larger than they are expected to, so you really don't know what that 9mm mold is going to do until you cast with it. It might work well in your 38/357, but the only way to know is to try it.
There are literally tons of used 38/357 molds for sale out there for very low prices... Lee molds are pretty cheap brand new.
If you asked nicely, you might even get someone to loan you a mold for a month or so.
I only size my bullets if actual shooting tests show that I get better accuracy after sizing. Most of mine aren't sized.