I feel your pain :)
Went thru the exact same problem around 2 years ago.
I went the CHUG way for him. It's a fantastic line for picking up all matter of G1 bots. My boys are also primarily G1 toon watchers with a little rescue bots also.
Definitely check out eBay and members sales here. Thanks to the kind ppl of the boards I have got a bunch of stuff misb for birthdays and Xmas at good prices.
My 6 year old is on his way to having the bulk of 84/85 characters. And we haven't gone near Daddy's toys for ages hehe