Even household items can be re-purposed. I use straws painted black for pipes. Small lengths of fine wire coiled up. I use small pieces of tinfoil to represent scrap metal (painted and weathered to add age) I also bought some little wooden things (look like drawer pulls) from a craft store. I painted them terracotta they make great pots for my Durbar display.
Hi
I get a lot of bits and pieces in plastic from the Nemrod range also look at the Historex range at
http://www.historexagents.com/shop/hxshop.php
If you want metal bits an pieces John Eden studios does a massive amount of stuff but you would have to email him and let him know what you are looking for
this is his website
http://www.johnedenstudios.com/
None of these accessories are painted you would have to assemble and paint yourself
Regards John
Scott and John, excellent tips, you remind me to add some more things:
Wooden coffee stirrers can be used for planks and boards in various scales. In 54mm / 1/32, they are a good size for clapboards to side a house, or in 1/48, too. They can be used to make the planked hardstands the Luftwaffe used at their forward bases. Wooden popsicle sticks can be used for similar purposes. Stirring sticks can be found at many grocery stores, popsicle sticks can be found, well, in popsicles

or in bags at the craft stores.
Foil, very good point, save the foil from wine bottles. Ask your dentist about the packs that X-ray blanks come in, he may be willing to give them to you (my dentist gave me a bucketful of them). I also use metal foil from candy to make flags, or straps.
Chinese take-out--I salvage the wire handles on the little cardboard containers. I also save the plastic containers that some restaurants use. Cleaned up, they make great storage or organizing containers (not so much a diorama tip as a general tip).
John mentioned John Eded Studios; Mr. Eden has the molds for much of the old Rose Miniatures line put out by Russell Gammage. That made me think of Forty-Third, Ltd, who have most of the molds for the old Phoenix Model Developments line. Their website is:
http://www.forty-third.co.uk/index.htm
and it's worth a look.
Now do let us see what you're building!
Prost!
Brad