How can I get additional Slave Select pins?
My solution would be to keep it simple. Use real pins for every SPI SS. Buy an other Arduino board with more pins if you need more pins.

What else is connected to your Arduino board ? Perhaps it is easier to reduce those pins. Almost all analog pins can be digital pins as well. You could use external ADC modules. Perhaps replacing 1-Wire or DHT11 or DTH22 with I2C sensors. Using a I2C display. There are many options to use buttons and leds with a few pins. And so on.