This is a particularly useful website
http://wiredinitiative.com/index.htm
this book is excellent
http://books.google.co.uk/books?id=3ShJAZUCuXEC&pg=PR7&lpg=PR7&dq=andrew+macbride+substance+misuser+trudy&source=web&ots=Nogr5paCFS&sig=045NAc_VONbGvfifj1V3JLFeN9o&hl=e&sa=X&oi=book_result&resnum=1&ct=result
http://www.exchangesupplies.org/
this site proide what might be a startling insight into using harm reduction materials and techniques.
It would help if you gave some more context on the nature of the agency / your placement