There are plenty of sources of "known" steels. GFS is a good one, as mentioned above.
Ground Flat Stock, aka "Gauge Plate" is O1 steel, available precision ground to various widths and thicknesses from any decent engineers suppliers (there are not so many around nowadays), and BS1407 "silver steel" is a water-hardening steel available in precision-ground rounds. Both have tended to be used for shop-made tooling and gauges. The precision grinding is an expensive process, which is just a nuisance if you want to forge, but it can be an affordable source of known steel if you buy old stock.
GFS have branched out from just supplying Ground Flat Stock and Silver Steel to include a range of materials and supplies for knifemakers, hence their listing of other knifemaking steels. Their "Silver Steel" is now to a slightly different spec: W1.2210, rather than BS1407. W1.2210 has a trace of Vanadium in it and is arguably better for knives.
Silver Steel is hard to move under the hammer, but seems to heat-treat well without needing a soak at Austenitizing temperature, something that can be difficult to achieve with backyard Heat-Treat methods. It is available in 3' imperial lengths and 1m metric lengths. If you are a beginner, these are great: you can hold the cold end and work the hot end without having to use tongs (which are the work of the devil). The no-soak and quench-in-water HT is helpful in keeping equipment costs down. Tempering can be done in a domestic oven, though you'll want to clean the oil off thoroughly if you oil-quench in order to minimize domestic disharmony. A lot of domestic ovens have quite wild temperature swings, so burying the blade in a tray of dry sand (I'd strongly recommend kiddies play sand, as it is washed, kiln-dried and doesn't smell of feline waste when heated).
If quenching in oil, note that American references to "Canola" oil are for normal Vegetable (rapeseed) Oil over here. As I understand it, when Rapeseed oil took off as a crop, the Canadian Oil Seed Growers Association decided selling Rape to American housewives wasn't going to go well at all and trademarked the unambiguous "Canola" for marketing purposes.
If you are looking at recycling previously-used items, you'll be looking at using "unknown" steel. There are lists of steels that may have been used for certain things, but it's not safe to assume that they were the steels actually used without a lot of extra research and/or experiment. 15N20, for example, is certainly used for bandsaw blades, but so are several other steels. It does not follow that a random bandsaw blade is made from 15N20.
It's usually a question of time vs money: if time is the limiting factor, spending money on known steel is probably worthwhile. If money is the limiting factor, spending time trying (and often failing) to get unknown steels to work well may be worthwhile.
If unknown steel is the way you want/need to go, old files tend to be a relatively safe bet. They tend to be a fairly plain high-Carbon steel. In the UK, look for Stubs brand in particular at car boot sales and the like.
