I'm 5'0 tall and also recently gained some weight, so I am in a similar boat with you. One detail you left out is what is the dress code at your office? That can make a difference, for example, I am in high tech marketing so I can get away with adding a jean jacket or other casual option, rather than a suit blazer.
Here's where I've had the best luck finding suitable petite toppers:
1. F21 - can be a goldmine if you are on a budget, and also petite. I bought my trusty pleather cropped moto there, for something like $10. It is still going strong after all these years.
2. LOFT - is where I got my petite white denim jacket. I have also had good success in the past with smart-casual suit jackets in petite sizes.
3. Lord&Taylor - has a wide selection of petites, suitable for work, and carries a range of brands/price ranges. I love their store brand cardigans in particular, and find the quality & fit is much better than Nordstrom's Halogen & Calson lines. I have this one but with gold buttons -- http://www.lordandtaylor.com/w.....lsizes-pet
4. Banana Republic petites - great quality jackets and cardigans, although may be higher price point than you want, but watch out for sales.
That said, the most versatile blazer I own is Theory. I did get it on sale, but even if I'd paid full price, it'd have been justified by the cost-per-wear. The Theory blazer I have is just so well made, slims and flatters, keeps me warm, has stood the test of time, and the charcoal color makes it a piece I can wear more than once a week and nobody is the wiser. I would not have invested this many $ in a bright red blazer or other stand out color/style. But just saying that for the right piece the investment can be totally worth it.
As for cardigans, once I find one that I like -- one that fits just right for skirts/dresses/pants, and is good quality -- then I duplicate. In black, cream, and a pop of color.