The BD242 characteristics are not particularly critical in that circuit. It is a PNP, medium-power transistor. You should look for alternatives that have similar voltage and current ratings in the TO-220 case with the same pin assignment,i.e., BCE from left to right looking at the "front". There are lots of possibilities. The requirements for the case and pin assignment are because you have already made or plan to make the same PCB pattern developed by BigBoat and JMJ. The other characteristic to look at is the Hfe, which essentially is the amplification factor. Most circuits are not particularly sensitive to that factor. The BD242 Hfs is 25 at 1 A and 10 at 3 A. A close match to those characterisitcs is the TIP32 (45V rating) or TIP 32A (60V rating). DigiKey (and probably the other vendors) allow a part lookup by characteristics from its web page. There were 9 pages of similar parts (i.e., same or greater voltage and current ratings in the TO220 or similar package) in the DigiKey catalog. The TIP32/32A was just the first one of that long list that was listed. Try Radio Shack. Although their selection is very limited, many of the parts it carries are common generic substitutes. It has a TIP42, which is similar, but not as close a match as the TIP32. You might try it just to see.

Many radio stores have a catalog that contains recommendations for substitute parts. I believe BigBoat has also given alternative part numbers that can be used.

The biggest problem with using a substitute part is when the circuit doesn't work right from the start. It could be the part, or more likely, another problem in the circuit. Unless you have trouble-shooting equipment (multimeter and inexpensive oscilloscope), it can be hard to find the problem. Good luck. John