I've got a 20" one lined up for my little-un in the shed, ended up getting her a new Ridgeback Honey 16" wheeled bike earlier in the year as I couldn't find a decent second hand one. It's been great, stabilisers came off last week so she's been tearing up the local park this weekend... not bad for a 3yr old.
I think the Honey was just over £100 posted which isn't too much, might be worth a look.