import random
import numpy as np

#numbers to sample from, could be a more complicated distribution
list_of_numbers = np.random.rand(1000) * 50

valid_samples = list()

while(len(valid_samples) < required_length):
    sample = [random.choice(list_of_numbers) for _ in range(100)]
    if (sum(sample) < 1000):
        valid_samples.append(sample)