They are all over the place.
Some of them you get for finishing missions in weapons master mode.
You best bet is to beat weapons master mode, I like to beat each level at least three times.
I'll tell yoy why;
After you beat it the first time, you should have all shops available except one.
If you have been beating each level three times your level should be pretty high by now.
Now go back and beat weapons master mode again, make sure and beat each level twice, this is a requirement!!! If you have followed my instructions your level should be around 72(Iron Edgemaster), which is what you need to unlock Subchapter 2.
Once you've hit that landmark, go beat Stage 3 of
Chapter 4 (even if you've already done it, you need to beat the level again).
With that done, Subchapter 2 will be all yours.