If you live in the US: not a huge fan of Walmart but the Sam’s Club plus membership ships a great diaper for a reasonable cost (Pampers and Huggies are good but $$) and the membership paid for itself in diapers alone.  Worth it if you live close enough to get the gas too. 

Good luck buddy - it’s an awesome trip and you can look forward to a 3 yo asking to buy money (Bitcoin) nearly every night!