Good luck - hope you find one!
I'm in a similar boat. Nate got a Lego Duplo Thomas the train set second-hand from a friend of mine when he was still a baby. I'm looking for some of the other engines/parts for it and it's been retired for a few years. I have some bids on some online, but most go for way more than I'm willing to spend! But he LOVES Thomas - sleeps with him (yes, the lego one!) and brings him to the dinner table and all that. Hoping I can get at least something to add on for Christmas. Definitely keep an eye out on ebay and stuff though - sometimes something will slip through without getting too expensive.