I’ve purchased my mylar at pleasanthillgrain. Scroll down about half way. They also have the O2 absorbers.
http://www.pleasanthillgrain.com/plastic_bucket_5_gallon_buckets_6_container_gamma_lid_lids.aspx
They only carry two sizes but they’ve met my needs, 1 for the buckets and one for smaller items.
If you want more choices go here - http://www.sorbentsystems.com/mylar.html but be careful to get the food grade ones.