I know some people may not agree with this, but eBay often have lists for parts boxes and "junkyard" models. Granted, some of them may not be cheap (with shipping and handling), but there's those who are selling parts boxes (mainly for cars and trucks, but there's often some that are for different types of kits). The best keywords to use to find such parts boxes are "model junkyard parts." It pulls up parts boxes, models that were built but were destroyed over the years (often listed for parts or restoration) and parts that were built, but not completed for the model's build. You can also use "model parts box" and "(insert scale here) model parts box" to get various different results.