The most obvious accessory for double bass is strings. Upright bass strings were originally made of animal intestine (called gut) but steel has become the standard material, although some musicians still prefer gut strings. Synthetic nylon is also an option. Gut strings are more fragile and susceptible to climate variations. Steel strings tend to produce more consistent intonation all around the fingerboard.
Reverb also offers double bass stands from companies like Hercules and NS Design, cases, and bows.