I'm with you on that. These bargains don't exist - or at least aren't easily found by word of mouth or advertisement. I struggled to find a 4yo for £4k so I settled for a 3yo.
I just bought a 5 year old tbx 15.3hh, has got all the basic flatwork and has jumped round british novice, and schooled over intro xc with no problems. He was under 3k, but had to travel over 100 miles to get him and hes really under conditioned, but i suppose that's why i got him cheap!
Check out horsemart, thats where all the cheap ones are