The best and cheapest way to get them is to order in bulk online really, the most expensive part is shipping but if you buy enough it's really worth it. I'm not sure how good an option this is for you right though sense he is so small and would need to size up pretty soon I would imagine, but when he is a bit bigger this is a really one of the best options.
For you I would recommend looking for reptile stores near you and see if they sell feeders or see when the next reptile expo in coming your way and buy some feeders there.
As for size, this is the chart most everyone here uses
I really wouldn't be too horribly worried about him skipping a few meals however. As long as he's eating the right size and looks healthy it won't hurt for him to miss a few meals, and petsmart mice are overpriced but are okay if that's your only option. That's what I fed my boy for the first few months as well until I found a better supplier near me.
Sent from my LG-D690 using Tapatalk