Where can I find a plastic box that is approximately 1.5"L x 1"W x 3/8"H?

To fit a tiny pcb inside.

 

Look at the usual suppliers like All Electronics, Goldmine, MP Jones, Radio Shack, etc.

Note you can also make your own boxes from flat plastic stock. Get some acrylic or PVC and glue it together. PVC glue can be gotten at home centers and methylene chloride is the glue for acrylic. If you make nice smooth edges, acrylic can be glued quite successfully.

 

You could repurpose an existing box. I've used one of these which is around the size you mentioned. I managed to get a cr2032 cell in a holder and a tiny PCB inside.
If you are in the UK you can get them at 7dayshop as well.
http://cgi.ebay.co.uk/3-LED-Mini-So...771?pt=UK_Garden_Lighting&hash=item414d84b82b

 

I got some tiny plastic boxes very much like the one you described at Mouser. They look so much better than any home-made thing I could make out of flat pieces!

 

Check out Hammond Enclosures website, then find a distributor, like digiKey, Allied, etc.

 

http://www.okwenclosures.com/

Ken

 

It may be a little bit too big for your project but the mini-altoids tins are probably close to the size that you are looking for. Just line the inside with electrical tape to prevent the metal from shorting out your circuit.

 

Hammond 1551 is the smallest of their plastic boxes.
http://www.hammondmfg.com/scpg.htm

Serpac C2 is the smallest of theirs.
http://www.serpac.com/c-series.aspx

Bud HH34XX series are their smallest.
http://www.budind.com/view/Plastic+Boxes/Hand-Held+Grabbers+Style+D