The toshers were just one group among a much larger collection of mid-Victorian working class professions surveyed by the pioneering journalist-cum-sociologist Henry Mayhew, at first on commission from the Morning Chronicle, but eventually in the form of his highly influential four-volume masterpiece, London Labour and the London Poor. Mayhew devoted some years to collecting statistics and conducting a huge quantity of original oral interviews with informants from the various groups that he divided the working poor into, and it's not only because of him that we know of the toshers, and something of their world – it's really also because of him that we distinguish between toshers and various closely-related forms of dragging a living from a harsh city; his "mudlarks", for instance, are actually all but indistinguishable from the toshers, and its arguable that, in attempting to impose order and enforce categorisation – a very mid-Victorian thing to do – Mayhew effectively created a distinct profession where none, perhaps, actually existed.

At any rate, we're not, so far as I'm aware, familiar with any specific parallels from other cities. This may be because the precise conditions that allowed Mayhew's informants to exist –  a large city with an extensive sewage network that was linked to the surface via gutters and multiple gratings, and was accessible through unlocked and unguarded outlets on a river – were not paralleled elsewhere. Or it may be that other cities had populations of people like the toshers, who simply went unrecorded – as London's toshers might well have done, absent Mayhew; there are very few references to them at all outside the pages of London Labour and I have never come across any that antedate Mayhew's work.

Anyway, the toshers' own answer to your other question – concerning their ability to make a living, in their hundreds, from scavenging the London sewers – really boils down to a couple of things. London was the largest European city of the day, and also the wealthiest; hence, there was more chance that things would be dropped and washed down into the sewers in reasonable quantity. And the toshers themselves were experts when it came to knowing precisely where valuable objects tend to get trapped within the sewer network. Without the happy combination of the two circumstances, it would have hard indeed for anyone to make a reliable living from this sort of work.

“Sometimes,” Mayhew wrote, “they dive their arm down to the elbow in the mud and filth and bring up shillings, sixpences, half-crowns, and occasionally half-sovereigns and sovereigns. They always find these the coins standing edge uppermost between the bricks in the bottom, where the mortar has been worn away.” He estimated, via a typically Mayhewian series of calculations, that the total of valuables washed into the city sewer network must run as high as about £20,000 a year then; $3.5m a year today.