Complete rifles without the mum can be aquired for around $100 if you're patient. I bought what turned out to be very nice Type 99 for $90 at the same auction. Has a ground mum.

Ideally, you want to match the parts to the rifle you have. Look for the small arsenal marks on it and try to match replacement parts to them. If you only have $40 in it, you got a lot of wiggle room, if the bore is ok.

Ebay is a source for stocks and parts, there are other sources out there too. The stock is your biggest cost but I see them in the $80 range quite often. Don't buy the first one you come across unless the price is right, others will come along for the price you want.