If you're in need of an interior door replacement, then you do indeed have quite a few options to choose between. The first thing to do, is of course to measure your door opening and see if you're working with a standard size door or something smaller or larger. Next up, if you want a matching look inside your home and you like the rest of the doors inside your house, then it can be a good idea to try to find something similar to what you have. Here it can be worth it to take a picture of your current door which you could bring to the store so it would be easier to get a good match. Of course, if you don't care about that part, then you can really start from scratch and carefully think about what you would like. Perhaps a nice carved wooden door would be a nice addition? Or if budget is a concern, then there are many other nice options on the market that either have a wooden look, or that are painted. If you prefer old doors, then it can really be difficult to find something new that has the same weight and look and feel as an old door. In that case, why not look for used! Perhaps pay your local Habitat for Humanity a visit, or if you have some similar store nearby where they sell used doors. Often you can get really good deals this way, and you might end up with something that are of substantial better quality than what you would find in the store. Of course if you do go that route, then you might have to be prepared to do some sanding and painting, to get the door to a suitable stage. Well, other good places to look would of course be your local home improvement store or if you have a dedicated door and window store. Just beware, if you want something rather custom, then a well made door can be pretty pricey, however it is such a big part of a home, so in many cases it can definitely be worth it. 
Legacy Doors 36 In. 2 Panel Arch Unfinished Knotty Alder Interior Door Left Hand Swing
"Experience the beauty and warmth of wood with this home interior door unit from our Legacy Doors Premium Wood Door Collection. Made from select hardwoods and ready to finish, this door will give your home added beauty and value for years to come." 
Masonite 30 In. x 80 In. x 1-3/8 In. 6 Panel Textured Hollow Core Slab Door
"Molded panel series. There's no better variety available than our fine line of molded panel doors from Masonite. And they're an environmentally sound choice too! Molded panel doors from Masonite are more durable and resist warping, shrinking and cracking better than solid wood doors. These beautifully engineered doors are available in a smooth or textured finish with matching bifolds to suite any decor" These doors can be found at homedepot.com
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