I got a pair of well put together sturdy boots from Asda for my 3 yo for about a tenner. He loves them as they look the part and he's had no trouble with them over a couple of miles.
I like the idea of getting a decent pair, but apart from when we were in Ambleside I've had trouble finding anyone who does them, and I would like him to try them on before buying so I'm not keen on ebay. I think Millets do some kid's gear, but when I last looked they didn't have boots small enough for a 3 year old.
S.