Note that a BC106 quite possibly never existed - see "BC106 where are you?" note at end of this answer.
The most important parameters of small bipolar transistors (like your two) are
- Maximum allowed Collector Voltage ( Vc or Vce or Vceo)
- Maximum allowed collector current (Ic or Ic or ...)
- Minimum and typical current gain figures (= "Beta")
- NPN or PNP
- Package may matter
- Power dissipation may matter.
If you use a part that has at least the same Vc max or higher, at least the same Ic max or higher and an equal or greater typical current gain then the transistor will work acceptably in the majority of circuits. There are special requirements that are affected by a number of other parameters, but in most cases you will not need to worry about them.
You can find many of Digikey's transistors here
or you can start at the top and search Digikey's whole websitewith BC107 data sheet here.
You can use the same method for your 2N3634 once you know its parameters.
It's worth noting how the above transistor search was arrived at.
I just entered "transistor" (without quotes) in the search box at the top level, then clicked on "Transistors (BJT) - Single (13,797 items)" and it took me to the transistor search page above
The next most important parameter is possible "Ft" - the effective maximum operating frequency (although the transistor is of no use at that frequency). If you need to care about Ft then chances are that you should be showing us your circuit and telling us what you are ring to do and then asking for advice.
A good source (one of quite a few) of information on US available transistors can be had from Digikeys online catalog. It allows you to select Vc, Ic, Beta, Ft etc. Whether the parts there are of use to you depends what country you are in.
A useful site that allows you to search 20+ suppliers catalogs at once is www.findchips.com. Searching that site for BC106 returns
showing that 7 suppliers (probably) have it in stock - BUT it turns out that none of the stock are the transistor you want - the code BC106 appears in other parts numbers as well :-(.
Rather than try and track one down at this stage I'll note that
Digikey sells the BC107 = 45V, 100 mA, NPN, Beta = 200, 300 mA dissipation, Beta = ? here
It uses an old style metal TO18 case but the dissipation is lower than for most plastic cased leaded devices.
Using Digikeys transistor selector page as mentioned above and selecting transistors with specs at least as good as listed above and using either a TO18 or TO92 (plastic) leaded case, returned 51 choices. Of these the cheapest in stock in 1's is a BC337-40, NPN, 45V, 800 mA, Beta=250, FT=100 MHz, TO92 case, 625 mW dissipation. It is likely to be an excellent substitute for a BC107 in most cases.
Also suitable are versions of the MPSA18, ZTX692, ZTX694 & 2SC29250.
Note that in the above I've left out some details which are more liable to confuse than help at this stage.
eg Beta is usually specified at a stated current.
BC106 where are you?:
It seems possible that the BC106 has never existed. A BC100 did and BC107 on did.
Some circuits refer to BC106 but that does not guarantee it's existence.
It is very likely that if you use the BC337-40 or similar that it will do very well in the original circuit BUT seeing the circuit would be even better.
As an example of brain-fade and BC106's, this discussion from 2009 refers to a BC106 allegedly in a circuit diagram here BUT when I look, I find the circuit uses a BC107.
SO BC106 probably (but not certainly) does not exist. Some circuits refer to it but no locateable (so far) data sheets and an early 1970's selector guide (book) of mine does not have it. Circuit references are most probably in error.
For For venerable Phillips 1977 BC107-BC109 datasheet see.
1989 SGS Thomson datasheet here