When you're looking for a specialist corset maker for your underbust corset, make sure that they are offered at a higher price at a high end lingerie store. This is because the corsets are often custom made and take a lot of time and attention to detail. If the corset you want is cheap, then you may be disappointed if it doesn't fit well and you will only find out once you've actually put it on that it was not what you wanted.
If you're looking for a specialist corset that can also be used as an underwire bra, then make sure that you choose one that is a light weight fabric, like cotton. This way, the bra is comfortable to wear, but the underbust corset has not been made to look as sexier or as figure flattering. If you're going to wear a bra under your corset, then select one that has padding in the cups or a soft bra lining to prevent the corset pulling out.
Try to get a few ideas from friends before you approach a lingerie store for an underbust corset quotation. You could ask them if they know of a specialist corset maker, or have they seen any other good lingerie stores that can provide a quotation on your specific order. This way you will be able to compare brands and models and find the best possible underbust corset quotation.