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 old-standard lead may be seen labelled as either 1.1 and 1.2 mm, as well as 1.15 and 1.18 mm. In general there are no problems with interchangeability as these metric designations are only nominal, though we have recently come across some cheap off-brand lead that is slightly undersized (.043 inch/1.09 mm) which works in some pencils but will not retract in others.
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. We offer a limited selection of new and old pencil lead here. A much larger selection can be found from our friends at the Legendary Pencil (formerly Legendary Lead) Company.
"Slim" lead of 0.9 mm (.036 inch) was the dominant standard from the later 1930s to the 1980s. It is still widely available despite the continuing onslaught of ever-slenderer leads in the last couple of decades. Confusingly, a few pencil manufacturers 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, mostly 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 1.5 mm and 1.0 mm here, though the former is often out of stock; the latter will usually require a bit of adjustment to fit "M" nozzles (a thin coating of shellac or beeswax will usually do the trick). Many other unusual and obsolete sizes can be bought from our friends at the Legendary Pencil Company. 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. "H" lead is another nonstandard Victorian size at 0.8 mm, for which the best current option is to take standard 0.9 mm lead and reduce its diameter with fine sandpaper.
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, or using a sharpened metal tube to punch out refills from block erasers. Note that many vintage pencils (among others, Parker Vacumatic, Parker 51, and Esterbrook) use a 6mm eraser, while classic Eversharp pencils use a slightly larger 0.25 inch (6.35 mm) eraser. Duofold pencils use a larger, 9mm eraser. Pendemonium's list of sizes is a useful resource but not always completely accurate. We currently sell replacement erasers in 5 mm, 5.5 mm, 6.35 mm (0.25 inch), 7 mm, and 8.5 mm diameter here. When replacing old erasers, be sure to pry off and keep the original metal cage holders for reuse. If that is not possible, a workable replacement can usually be fashioned using metallic aluminum tape.