The best I came up with on an internet search was a couple of tent makers, but probably not the style of tent you have.http://welldressedtent.com/index.phphttp://tentsmiths.com/period-tents-options-pavilion.html
A good place to find odd plastic parts is a local Ace Hardware store. Show the store clerk what you need. I'm amazed what they have.
It looks like it might be easy enough to make either from wood or PVC pipe.
Another option is a military surplus store. Look for part number MIL-T-10035 or NSN 8340-00-242-7872. In military speak the description should be "Tent, Toggle, Wood, Door".
I hope this helps.