there re many sources for bags. uline, international plastics etc.
For 10 Lbs, that is about a 2 gallon bag, and at that size, id pay the extra few pennies and go with the 4 mil bag.
I put 10 Lbs of lead into 2 mil gallon bags but I wouldn't guarantee that'd stand up to postal abuse and any sudden tugs or whatnot may not end up well.
Will 2 mil work, yes but how often are you moving it, what are you using it for etc. If it's for coffee, something that will be opened and closed many times and hefted around, go with the 4 mil.
Aaron