You can easily find parts for your dodge d100 truck at both brick and moral store as well as in online stores. However, as you have mentioned that you want a place that has a large selection, so I will suggest you to go for online stores because they have huge auto parts inventory and you will be easily able to choose the part of your choice. Last month, I was looking for transmissions, so I was have a look in the inventory of automotix.net and there I saw dodge d100 truck parts. So you can have a look here.

Sources: http://www.automotix.net/

Well I would recommend you to check out an online store called autopartsfair.com, as they have giant inventory with more than 35 millions used and remanufactured auto parts. So just have a look at them, I am sure you will find the stuff you are looking for.

Sources: http://www.autopartsfair.com/exterior_parts/