Most amateur astronomers who select a dome purchase it ready-made from a manufacturer. If you go this route, you must decide whether to have the manufacturer install the dome or to do it yourself. This choice depends upon your construction skills and level of confidence you have to take on such a project.
Unfortunately, you can't just order a dome and be done with it. You'll have to construct the building beneath the dome first, with the "mating ring" made to the manufacturer's specifications. Four major dome manufacturers (as of this writing) for personal observatories are Ash Domes in Plainfield, Illinois, Technical Innovations (makers of Home Dome and Pro Dome models) in Gaithersburg, Maryland, Explora Dome in Litchfield, Minnesota, in Astro Domes in Yandina, Queensland, Australia.
Occasionally, amateurs build their own domes. This is not something to take on lightly. Certainly, if you have the skill, you can build a form for a single "orange slice" panel, construct the required number of panels, then bolt the sections together to yield a dome. I never contemplated this because getting the curvature so all pieces assemble into a smooth dome is no easy task.
Other builders have started with a form of metal rings over thin, curved girders. Usually, the material is aluminum. Over this hemisphere, you could stretch either plastic or a water-resistant fabric. If you choose fabric, seal the cloth against moisture, and apply resin in layers to add strength.
If you choose to build (or have built) a roll-off roof, like I did, I have some suggestions. First, be sure the roof is gabled (triangular) and not flat. A gabled roof will not allow rain or (much) snow to accumulate on it. You can save time and labor by purchasing pre-made triangular roof trusses, if you can find them in the right size.
Next, consider how you want the roof to roll, mechanically speaking. A number of caster styles are available in plastic and metal. Generally, casters roll over tracks or within troughs of metal or, in some cases, wood. Be certain your casters have a large enough weight rating for your roof load, plus snow, if any.For my observatory, I chose steel V-groove casters that rode on an inverted steel L-beam. I installed four casters on each side of the roof. That load capacity was more than I needed, but the roof rolled more smoothly on eight casters than on six. Remember to lubricate steel casters at least three times a year.
Be certain to design your roof so no rain gets in. Overhang the roof at least a foot in the three directions, leaving only the side that slides over the observatory at roofline level. On that side, observers usually install a soft rubber flap attached to the roof to cut down on dust getting in.
The roof must be secured when closed. I used four steel turnbuckles. In the southwestern desert, spring winds are often strong. The roof I build had no problems surviving wind speeds in excess of 60 mph. Other observatory builders have employed steel hasps, usually one in each corner.
Some amateurs, for aesthetic reasons, design their roofs so they overhang the track ends by a small amount (approximately a foot) when open. This shortens the length of the outriggers, the wooden rails attached to the angle iron the casters ride on. Others make the outriggers longer than they need to be so the roof rolls farther from the building. Such a plan minimizes the amount of sky the roof blocks in the direction of its opening.