Glass domes are fragile and come in many shapes and sizes. They are expensive to produce so sourcing replacements is not easy.
There are three main types of clock that have domes:
400 day "Anniversary" torsion clocks use circular domes and there are three common sizes. Miniature clocks use domes with a diameter of around 100mm standing about 160mm high. I usually have one or two available at £20 plus postage. The standard size clock uses a dome with a diameter of about 140mm standing 260 to 300mm high and I probably have five or more at any one time. New ones are light and whiter, with a rounded bottom and are £28 plus postage. Old original ones are often greener and heavier and cost £40 plus postage but they will have imperfections in the glass and maybe some nibbles around the bottom. Large domes for Bandstand types are around 190mm diameter, standing 400mm tall. These are rarer so call to check availability. They will be at least £100 each plus postage. All shipping is within UK only and at your risk, as couriers will not take responsibility for glass. By the way, I don't keep any perspex or plastic domes.
French mantel clocks and English skeleton clocks are the other types that you'll regularly see under glass domes. These are easily broken as they have to be removed for winding every week. But they're often impossible to match because they generally sit in a narrow groove in the wooden base on which the clock stands so you need the exact footprint. Even if you are lucky and find one to suit the groove in the base, it needs to be the correct height and shape to suit the clock. They will set you back over £250 so you simply must check for a perfect fit before buying, which means transporting the clock and its base to the dome supplier.
They come in such an endless range of sizes that I rarely have more than two or three for sale. An alternative is to find a dome and matching base and then have the base covered in a material of your choice and your clock and remounted on the new base. I have just two at present, one measuring about 300mm (H) x 400mm (W) x 220mm (D) priced at £200 including the bare wooden base. The other is 320 (H) x 270 (W) x 120 (D) on a good ebony base priced at £180. UK postage will be about £20.00
I have a contact who can repair cracks in domes but it's not cheap and inevitably it won't be perfect; it's worth considering though if the clock is an important one or you can turn the dome round so that the restoration is at the back.
If you need to replace a broken bevelled glass panel in a carriage clock, look here.