You're correct in that most reputable resin casters make stuff as they get orders because it does not pay for them to carry a lot of unsold inventory. My only experience has been with Modelhaus, Replicas & Miniatures Co. of MD, M.A.D., and Ed Fluck.
With these resin casters, waiting awhile for your order is usually the norm, but well worth the wait due to the quality of their products. Yes, it can be frustrating to have to wait for months for something that you want "now", but that's just the nature of the beast. I just placed my "final" order with Modelhaus and I'm pretty sure I won't be receiving it until November or December, but they won't bill me for it until they ship it...... It'll be my Christmas present to myself .