Bunk beds and lofted beds are a very good way to make extra floor space in a room. If two children are sharing the room, then a bunk bed will easily house both children while still leaving ample space in the room for a second dresser, toys, and more. If only one child is in the room, then a loft bed is still a great way to make room for the dresser or a desk under the bed instead of in the rest of the valuable leisure space. The problem comes with getting into and out of the bed on the top. Some beds come with bed ladders, while others have it built into the side as part of the design. Others do not have one at all. For those that do not come with one, it is best to purchase one separately for safety reasons. Falls from the top bunk or from a lofted bed are more likely to cause serious injury than a fall from a normal height bed.
When choosing, the design is entirely up to you. It can be straight up-and-down or it can be angled. Angled is easier to use, but straight takes up less space. There are a number of websites and home centers that sell them. Some can be permanently affixed with screws or bolts while others attach with simple hooks. In some cases, these same sites will sell parts to repair broken bits of yours in the event that it somehow breaks, such as can happen if children are playing roughly or if a greater load is applied than it can support.
If you are a handy person or if you know someone who is, then there is always the option of making your own. You can buy raw lumber and start from scratch, or you can buy pre-fabricated pieces and assemble at home. For the do it yourself person, often the easiest is a permanently attached straight ladder. This is not to say that you cannot make an angled one, but it is more difficult and does take more time. It is probably a good idea to purchase hooks if you plan for yours to be removable, because this part is generally inexpensive and would be time consuming to make in such a way that it is both functional and not too large.