Just about any bike shop will have SS freewheels. Shops that specialize in BMX will have the widest selection. Some shops may have spacers, or you can order them online. That said, the chainline should be almost perfect without them.

Forgot about 1/8": If you get a 1/8" freewheel, you'll need a 1/8" (single speed) chain. This will work fine with your 3/32" chainrings.