I would try microtools first if you know the size or can measure:

http://www.micro-tools.com/store/~/C...nd-E-Ring.aspx

if not you can try a grab bag from places such as ebay, or surplus shed:

http://www.surplusshed.com/pages/item/m2916.html

http://www.surplusshed.com/pages/item/m3481.html

You can search for optical screws as well on google. As for the plastic part, is it a nut you talking about? if it is not a common part, you may have to make it, or take it from a junker lens of the same model. good luck