I'm 5'11 210 lbs and wear a full back brace, it comes down about 3 inches past my hips and all the way up to my sternum. The front part is cloth with aluminum supports running horizontal, the back part is a rigid plastic material.

My problem is I cannot find a holster that will work with my brace on. any suggestions?

What kind of gun are you wanting to carry?

Here's a few that will work with your body brace -
Pocket carry
Thunderwear type
Bellyband large enough to go around brace
Dayplanner

I'm just talking about what could work with a brace... NOT what you can physically do.

Here's what I see as important considerations...

1) How do you get around on a daily basis, i.e. walking, wheelchair, walker?

2) Is this brace temporary or more of a long-term item?

3) Could you make modifications to this brace such as glueing a 2" velcro strap around the sides of the plastic
piece and modifiying the straps on a thunderwear or bellyband so they are easy on and off, instead of trying to fasten the belt all the way in the back?

4) Is ankle carry and drawing a physical possiblity for you?

5) For pocket carry, would you have to modify your pockets to accommodate a decrease in your reach?

6) How about one of those t-shirts from Kramer Leather with the gun/ammo pocket under the arm?  Do you have someone who can sew and make adjustments to this t-shirt OR make one of your own?  I figure you could probably wear an XXL or more likely an XXXL t-shirt over this brace?  The next issue would be how to make it stay in place against the plastic. If you can add something to the plastic to 'grab' the shirt material, then it wouldn't twist around your trunk.

7) Can you remove this brace for a while so you could drill a few holes, then poprivet a kydex holster onto it?  A better option might be taking a dremel and creating two slots for a 'belt' to fit through.  Make these far enough apart that you can now strap in a regular belt holster. You might also add on a 2" elastic strap to go over the body of the holster and hold it so it doesn't shift around on your 'belt'.

It takes imagination as well as understanding what's happening to your brace when you add or remove material(s) from it.
Remember, the brace is supportive only if it has enough structure and rigidity to do it's job.

As far as modifying the brace... if you do, will this prove to be a problem when going to your next checkup/p.t.?

Can you post a picture of the brace from the; front, back and both sides?

I really figure that any sort of holster against the plastic body will be hard to conceal.
The front sounds like a better place to fix a holster to as far as concealment.
You might be able to modify a fanny pack holster like a Tommy's Gunpack so it would ride securely up front, yet allow you to draw and reholster from it.
Since you have zero or very little trunk flexibility with the brace on, you're likely to be drawing and reholstering on feel alone with a stiff trunk to boot.
I know when I draw from a concealed holster, I twist my trunk and bend some too.

Good luck!

Quoted: I really figure that any sort of holster against the plastic body will be hard to conceal. The front sounds like a better place to fix a holster to as far as concealment. You might be able to modify a fanny pack holster like a Tommy's Gunpack so it would ride securely up front, yet allow you to draw and reholster from it. Since you have zero or very little trunk flexibility with the brace on, you're likely to be drawing and reholstering on feel alone with a stiff trunk to boot. I know when I draw from a concealed holster, I twist my trunk and bend some too. Good luck!

Modifying the brace does have the advantage of it being a medical device.  Any buldge formed by your weapon on the brace could be passed off as a medical moniter or other medical device.  I worked as and EMT for 4 years and saw many people who had braces on that had all maner of devices hung off of them.