Gary,
I spend a lot of time searching the internet for my models, at a price I like! Ebay is just one of the many sites I frequent. Generally I only buy from within the UK - postage costs increase rapidly from elsewhere. Sadly auctions are not as common as they used to be, but you can still pick up bargains, you have to be patient, and visit regularly. You're right in setting an upper price, as chasing prices is the quickest way to overpay.................... It's sometimes worth mis-spelling search words, as sellers can do this, and also post in incorrect categories.
I've signed up for newsletters from most retailers that offer this service, keeping an eye out for sales/special offers. I have a mental list of models I'd like to make, some of them have been on it for years & years............ John has mentioned damaged boxes from Hannants & Creative Models - you can also find them in Amazon 'warehouse deals', but they take a bit of searching for.
Dave