The USA version of DS is pathetic. My last order for 12 parts - that was the 12 they had in stock, not the 30 I started with - I got please reorder on 4 of them. So out of 30 parts needed, I got 8.
Anyway, back to your problem - when I go to make an order I go through playmodb.org and find parts that look like good substitutes and then rank them according to best fit.
So in your case for trailer turntable 30 21 6640 my list would look like
trailer turntable 30 21 6640
or 30 21 6600
or 30 25 1770
or 30 27 2350
or 30 04 8650
And when I'm on the phone with the DS person, I say, oh, you're out of 30 21 6640, how about 30 21 6600 which looks like a replacement. That way they don't complain when you order lots of parts.