You can probably get a decent enough one that will go up to 5000 grams at Walmart or Target for $20 or less. Just make certain it has a "tare" button and you're good to go.
also when should I weight him? 2 or 3 days after a meal?
The most reliable weights are taken right after they defecate (their "empty" weight). Weighing them after a meal won't give you an accurate picture of how much they're growing, as they have a slow metabolism and take awhile to digest their meals.