Go to OC Farm Supply and get yourself some 40 lb bags of pumice for about $12 each. Tell them you are a member of the Orange County Bonsai Society and you get a discount.
Go to Green Thumb and get yourself some pine bark micro nuggets. If my memory serves me correctly, they stock both mini and micro sizes - you want the micro. They should be very small pieces - about 1/2" and less.
Add the pine bark to the pumice at about a 10%/90% to 20%/80% blend. Finished!
Avoid adding any potting soil or similar organic product to your bonsai soil that will clog the soil. If you water your bonsai and the water doesn't immediately pass through the top of your soil and flow out through the drain holes (leaving wet soil particles behind) your soil is too dense / clogged.