The standard lead diameter from around 1915 up until the 1930s was .046 inch. The exact metric equivalent would be 1.168 mm, so this older standard lead may be seen labelled as either 1.1 and 1.2 mm, or even 1.18 mm. In general, there are no problems with interchangeability, as these metric designations are only nominal.
Old .046 inch lead is often found at antiques shops and pen shows. In the USA, Scripto still sells a 1.1 mm lead pencil, along with refills, and Sanford sells the "Sphere", another pencil that takes 1.1 mm lead; both are widely distributed. Another brand that still uses the old standard thickness is Yard-O-Led; their rather expensive refills should be available from larger pen stores. Autopoint is an American manufacturer that sells both thick lead and the pencils that use it. We offer a selection of Autopoint pencil lead on our Nibs, Parts and Supplies page.
"Slim" lead of 0.9 mm (.036 inch), used in most pencils from the 1930s through the 1960s, is still widely available, despite the continuing onslaught of ever-slenderer leads in the last couple of decades. Confusingly, some pencil manufacturers have recently started to advertise their 0.9 mm pencils and refills as 1.0 mm, even though not one is in fact a true 1.0 mm.
Larger leads, used in special marking pencils, may be slightly more work to find. The most common sizes were 2.0 mm (.075 inch) and 3 mm (.120 inch); broker's pencils typically used 5.5 mm (for which one can use Montblanc "Sketch Pen" refills). 2.0 mm lead is still a standard size and easily found at any art supply store, especially one that carries drafting supplies. Jerry's Artarama carries both 3 mm and 5.6 mm lead. 2.5 mm lead is still made by Koh-i-noor, though it is not widely stocked -- a size that was in more common use in the 19th century.
Lead to fit Victorian-era pencils can be extremely difficult to find. The two most popular sizes of the 19th century were 1.5 mm and 1.05 mm ("VS" and "M", which originally denoted both thickness and hardness: Very Soft and Medium, respectively). We sell both 1.5 mm and 1.0 mm here, the latter a nonstandard size that will fit "M" nozzles with a bit of adjustment (a thin coating of shellac or beeswax will usually do the trick), as well as 2.5 mm -- another scarce Victorian thickness. Faber Castell makes 1.4 mm lead, but this is a very uncommon size in older pencils and is not close enough to 1.5 mm to be substituted for it.
If the information above has not helped you, check with Jim and Jane Marshall of the Pen & Pencil Gallery in England, who can supply quite a few odd sizes upon request. Pendemonium is another good source for older and hard-to-find sizes of lead.
Replacement erasers are less standardized than lead, but one can usually find something that will fit -- even if that entails cutting down the long cylindrical refills sold at art supply stores. Note that many vintage pencils (among others, Parker Vacumatic, Parker 51, and Esterbrook) use a 6mm eraser; Sheaffer and others make refills of the correct diameter. Duofold pencils use a larger, 9mm eraser. Pendemonium is again a useful resource here.