Gun shows, personal connections (friends etc.), occasionally gun stores, and finally the EE here.
Getting a good example can be hard. Many of them are in pretty poor condition (Type 38s especially). Its just a matter of keeping your eyes open and paying attention so that when a good one comes up you can grab it. For example I remember about a year ago there was a beautiful all matching excellent condition early type 99 that came up on here with a Type 30 bayonet, and sold for 1500$ within minutes of being posted. Most aren't worth that much so don't think that is the average selling price, just a example to show good ones do come up from time to time.
The interest in the Arisakas has been rising in the last 10 years, ever since 'The Pacific' came out, and people realized they aren't just a crappy gun. Take your time, keep your eyes open and I am sure you will find a nice one.