We were told at our school's 11+ meeting, that the questions are selected from a bank of 1000 (I believe that was the number), and thus anyone in a position to look over the papers from year-to-year will begin to see questions re-used. If this is accurate, then this explains why past papers are not available, though I have heard rumours of tutors who might once have been in the school system, who have managed to make copies of old papers....
The nearest you will get to an 'old' paper is the set published by nfer themselves, which are available from all the usual outlets, including this web site. These papers, however, have 85 questions rather than the 80 which you get in the Bucks 11+. (I have also heard rumours that the actual 11+ papers are more difficult than the nfer practice ones. Has anyone any views on that?) The usual advice, however, is to save these papers for use late on in the preparation cycle. Patricia has posted in a number of places on this site about the best order to use papers in. I'm sure when she drops by that she'll post a link for you.
Assuming your child is in a state primary in Bucks, they they will get the opportunity to sit practice papers in school, under exam conditions, later in the summer or in the autumn. These papers are public domain, because the children get sent home with them, and they are re-used year after year. (I think there might be a few different sets of them. Does anyone know for certain?). Just be warned, that if you get hold of these and use them with your child, then they won't get a 'proper' practice of sitting papers in exam conditions prior to the actual 11+.