Need advice switching to buying hay in bales vs pre packaged

my little guy picks through his top quality hay, wasting most of it because he only likes certain pieces. i get this is totally natural behavior which is fine!! i just need help switching over to a more cost effective hay as a college student. The only reason i’m extremely nervous is because of the risk of pesticides or chemicals, since ive heard horror stories of bunnies dying after a hay bale was treated with something it shouldn’t have.

Where should I look? First cutting timothy hay is what i need, as his stomach is super sensitive, and he hasn’t done well with lower quality hay in the past. At the moment I buy from Small Pet Select. Additionally, how should I switch him over once he has the hay? Thank you all i’m just very nervous and want to make sure i don’t disrupt his GI tract.

edit: I looked at tractor supply shops near me but the 50lb cost effective timothy hay bales state that they aren’t meant for small animals and could potentially contain harmful weeds. my guy has no front teeth so that’s a definite issue☹️👎 the small pet hay bales on the same website are like 3x the price with a 20lb max. any ideas?