substituting transistors and diodes
Any two-terminal item (diode) may have a 1Nxxx part number, but only
three-terminal devices get 2Nxxx part numbers in the JEDEC part numbering
scheme.   So, the "2N4148" is an error, it should be "1N4148", which is
a common and widely distributed switching diode.   TIP121 is also
an easily available part, about $0.69 at DigiKey.com.    That
same supplier has 660 000 of the 1N4148 available.

taydaelectronics.com DOES have 1N4148 listed1N4148
and a similar part to TIP121 (80V, 5A) is its sister part TIP120 (60V, 5A) or
another sibling TIP122 (100V, 5A).  Both TIP120 and TIP122 are available at
taydaelectronics.com.
The data sheet for TIP120, 121, and 122 is available if you want to
examine the detailed characteristics TIP120-TIP122