If you don't really need entropy and simply want to get a different sequence of pseudo-random numbers on every startup, you can use EEPROM to iterate through consecutive seeds. Technically the process will completely deterministic, but in practical terms it's much better than randomSeed(analogRead(0)) on an unconnected pin, which will often make you start with the same seed of either 0 or 1023. Saving the next seed in EEPROM will guarantee that you start with a different seed each time.
#include <EEPROM.h>
const int seed_addr = 0;
unsigned long seed;
void setup() {
seed = EEPROM.read(seed_addr);
EEPROM.write(seed_addr, seed+1);
randomSeed(seed);
}
If you need real entropy, you can collect it either from clock drift, or by amplifying external noise. And if you need a lot of entropy, external noise is the only viable option. Zener diode is a popular choice, especially if you have a voltage source above 5-6V (it will work with lower voltages too, but produces less entropy):
The amplifier output has to be connected to an analog pin, which will produce several bits of entropy with each analogRead() up to tens of MHz (as fast as Arduino can sample).