Where can I find White Spacer?

Krylon makes a spray for plastic that works great and is totally dry in minutes. Comes in LOTS of colors.

So now you can stock that clear, and make what you need.
YOu can also use clear, but put white tape on the inside of the frame.

If it the taller profile, you can place white matboard (or even a color to match the backing).

Baer mentions the white paint option.

But here's the nifty thing. Spray the backside of the clear. The side that faces the frame. Leave the clear side a clear. The white will look better that way. It won't look painted. It will look like white plastic. More depth.