Can anyone find SQUARE 90degree fittings for vinyl post?

Hi everyone.

I've been searching forever for a place where I can get square 90degree angle fittings.
I'm trying to fit together a ring of 6" vinyl fence post as seen here: http://www.homedepot.com/Building-Materials-Fencing-Posts/h_d1/N-5yc1vZbls5/R-202084745/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

this is a 5", but they have 6" at my local store.

Anyways, I know they're not meant for this, but does anyone know where I can get 90degree fittings? All ive seen so far are endcaps.

I'd like to use square instead of round pvc because it's flat and I could fit bulkheads on the bottom.

So does anyone know where to find fittings? Or somewhere else that would have square tubing with 90 fittings available?
thanks

That's not how those fence posts work so I doutb you will find any... what are you trying to use them for?

I know this post is a few months old, but in case you're still looking or someone else is looking for an answer, have you considered rain gutters? You could find a corner piece to match your fence or just use all gutter and cover with panda film. You would need to make sure the gutter is deep enough and to support the net cups well, but it could be an option.