Finding shower door replacement parts usually involves calling the original door manufacturer or visiting the store where the door was purchased. You may not have a choice in which parts you choose unless you are altering the door, although if it is glass that needs to be replaced, this can often be customized to fit your needs. There are companies who specialize in glass replacement, so you should contact one of them to find out whether a new glass door can be installed into the existing frame if that is what is needed.
For the most common shower door replacement parts, you will likely not get to choose which options you need. The manufacturer of the door will probably sell hardware, gliders, and other items to specifically fit your door. Other items, even if similar, may not fit or function properly. You may be able to find such items at your local home improvement store, or by visiting the same retailer or provider who sold you the shower door. If they do not have the parts you need, the original manufacturer may be your only option.
If the shower door replacement parts you need involve the actual body or glass portion of the door, this may be more customizable than the hardware. You may be able to have a new glass door created and installed to coordinate with your existing door frame. To choose the right option, make sure the glass you want is made for shower doors.
There are some companies who specialize in making custom shower enclosures, so you will likely find a wide selection of glass samples there. You will need to specifically ask if they will insert the new glass into your old frame. This is often not much cheaper than buying an entirely new door, and may even be more expensive since you will have to have the glass custom-fitted to the dimensions of your frame.
You may also be able to get glass shower door replacement parts by speaking with a glass repair person or installer, but one who does not necessarily specialize in shower doors. Choose glass, making sure it is tempered and the required thickness for added safety, and then ask about prices for installing it. These services may not be available everywhere.
Generally, if the glass in your door breaks, it is most reasonable to buy a new door. Shower doors can be purchased at relatively low prices, and some would likely be cheaper than even the most inexpensive replacement glass and installation. If you want a new look for your bathroom, have a new door designed which meets your criteria. If you aren't sure which part needs to be replaced on your door, ask a trained professional to help to avoid causing further damage.