Trade show
I have researched and bought from many companies in the hardware arena.
Here is a non-comprehensive list off the top of my head:
Testrite
Expand
Mark Bric
WS Display
Orbus
Duo
Brandstand
Drytac
Bowman
If you have done a Google search for "retractable banner stand" - you will get thousands of hits for sites/companies that just resell Orbus products. They all use the same pictures with the same model in them - so don't expect to maintain any price competitiveness with Orbus. Not saying anything bad about them, but if you call them and tell them you have a printer they will make you a dealer and send you the dealer price list. There is very little qualification to be a dealer for them - so everyone carries their stuff.
Places like Creative Banner and Signs+Banners just resell other guys stuff but call them something different.
You have to know what quality level and usage expectations are coming from your clients. If they want to just put a graphic in a retractor and leave it up - you can get a $16 retractor from Orbus (their DFA brand) that will work fine. If you have the need to change out the graphics on a regular basis - your options get narrowed. I recommend styles that have Velcro at the bottom and snap rails at the top. Re-taping graphics into banner stands is not fun - especially if you are by yourself. Really no fun when once a year you let the little key slip out and the whole thing flies back into the stand and you have to take it apart to fix it.
Of all the players - Testrite has the best tradeshow stuff, but a very narrow selection for retractors. Orbus has a nice selection, but remember the caveat above. Expand is top notch stuff and has a graphic cassette system for one or two of their retractors.
I'm not a big fan of buying stuff through distributors - so I try to stay with the guys that actually make the products and will sell directly to you.
I'm sure others can chime in and offer more vendors that I have missed