I can however give you some good guidelines to help point you in the right direction. What you require are a pair of wide legged, flat fronted khakis or chinos in a relaxed fit.
Pleats and cuffs are perfectly appropriate with polo, but are not ideal for a shorter chap such as you as they detract from the length. In order to give the greatest visual appearance of length and height, you will need to have flat fronted and uncuffed trousers. Flat fronted pants are also more slimming; however pleated fronts are a little dressier. On this occasion though, I believe you can make up for the dressiness elsewhere, and go for the flat fronts.
You should also pay close attention to the back pockets of the pants and try to choose a pair without pockets (if you can find any), or with slim pockets. Either way, go for the smallest flattest pockets at the back that you can. Big pockets emphasize the rear of the trouser, and as you mentioned that you have a generous rear, it’s best to downplay it.
In terms of length, the pants should break at the laces of your shoes (which incidentally I recommend should be loafers or suede oxfords) and sit just above the heel at the back.
I would go for a black polo, tan/clay/stone colored chinos or khakis that are wide legged and flat fronted, small pocketed at the rear, and have no cuffs, and team it with a black belt and black loafers or suede oxfords (with, of course, black socks!)
If you wish to up the smartness a touch, you could also team it with a smart sports jacket or blazer in a dark color, which you can then of course remove and offer to drape across a lady’s shoulders should she start to feel chilly at the event! Always think ahead!
In terms of stores to try, you stated that the Banana republic are not ideal. Perhaps you could try Gap or Dockers as an alternative.
If you wear a nice pair of black or tan pants, you can wear a number of different things on top. For the winter months, go for a nice pair of wool slacks and for warmer months you can opt for thinner fabrics.
On top, you can wear a simple button down shirt that is a bit more relaxed that you would usually wear to work. A flannel shirt, when done right, can look casual and chic, yet still professional for work. Dress it up a bit with some nice black dress shoes and you’ll be good to go! Add a blazer if you feel you look a bit too casual.
A sweater on top is also a nice look. A nice rich color (like navy blue or burgundy) would look great on you. To make the suits you already wear look less stuffy, why not wear a t-shirt underneath? Make a solid one, again in a darker color. The result will be a youthful look that is also professional
A final key point is that it is your grooming that really dictates how smart you will look, not your clothes. By all means ensure that your clothes look smart, but the key to success is to ensure that you shower, shave well, and pay attention to your hair before you go out! Also, make sure you feel comfortable in what you wear; if you feel uncomfortable it’ll show. If you feel comfortable and well groomed you’ll feel relaxed, you’ll smile more and feel more confident. THAT is what will make everyone stop and do a double take when they see you.