Sauerkraut juice, ACV, ginger tea, betain HCL, digestive bitters and lemon are the best for kick starting stomach acid production.
Stomach acid is more important than digestive enzymes.
Here is a quote from GAPS book:
If we can restore normal stomach acidity then this stage in digestion should go without any problem, because the stomach acid would trigger the pancreas to produce its own enzymes. That is why I consider restoring the stomach acid level far more important than supplementing pancreatic enzymes.