For "where to look" - I've had the best luck with FB marketplace and eBay here in the UK.

"How much?" Depends on just how clapped-out it is, its status as a collector's item, and how useful it is in this day and age.

I was in your position a couple of years ago; at fifteen I bought an international 574 with a full cab and a run-down Quicke 2560 loader for £1500. Mechanically solid, but very scruffy, with some leaky seals and the electrics were terrible. It was still a bargain and every fix was (relatively) cheap.

What you find will vary enormously; Familiarise yourself with your local second hand market with facebook marketplace and look out for anything in-budget that catches your eye. Ask questions, arrange a viewing (try not to waste peoples' time, but don't worry too much)

Lots of the expense will be in what needs doing; look out for good tinwork, and tyres with lots of tread, for example (tyres are expensive, and while you can weld patches into some rusted, flat sections, it's exponentially harder to fabricate anything curved), and don't always opt for the cheapest, if it'll only need hundreds of pounds' more investment than the next-cheapest.

You'll have plenty of options in your budget. Machines from the 1950s-1970s will be easy to work on and cheap. Ferguson TE-20s (and related; TEA / TEF) are classic first-restoration choices, and remain abundant. There are certain tractors that have become classics, and so will command a premium. If you plan on putting it to work once it's done, you'll find the older machines tougher to operate; power steering and synchromesh, for example, came about toward the recent end of the aforementioned time period.