Depending on what size I need.
A couple years ago, I saw a bunch of bike inner tubes on a clearance table at wal mart. I picked up a couple sizes, and still have over half of each of those in a box.
Also, road bike tires are very thin. ~1” or so dia. And I try to swap out inner tubes whenever I change or remove a tire. (Flat or not). So keeping a couple of those Round is easy. The small 1” tubes work great for taming the small straps like harness chest straps and even shearwater elastic arm bands.
Ask anyone you know who rides road bikes a lot, and they will probably have some used tubes to donate.