Depends. I recently was in that situation and it was easier and more cost-effective to just print them.
I recently bought some Lego Star Wars sets and printed out some Display stands for them but the connection between the stands and the model was expected to be a 2x4 Lego plate. I didn't have those plates at hand so I looked online and found it from the official Lego site.
The individual "Plate 2x4" would cost 0.14EUR each. Since I needed 3 this would be 0.42EUR. But the mailing costs would be over 9EUR.
So ordering 3 of those Lego pieces would cost me almost 10 bucks. I just printed them out which worked well, they were a bit tight fit but are still holding.
But I wouldn't necessarily say that this is a replacement for actual Lego pieces. As a quick alternative that you can't see or that has less interaction with other pieces (doesn't need to fit correctly on all sides) then I think this can work.