I am changing a 40A 2-pole (240V) Federal Pacific Electric Co. circuit breaker but I can't find one. Can I use a 50A 2-pole? It's for a wall oven.
REPLACE THIS PANEL NOW
Your panel almost certainly is suffering from breaker-to-busbar contact damage, rendering it a ticking incendiary device.
See this answer for the gory details on just what is wrong with FPE's "breakers".
If you post a picture of your panel (dead front off), I can determine if you can have your electrician install one of the Eaton retrofit kits (the kit itself is several hundred $, but it saves significantly on labor as the existing enclosure can stay in place) or if you need a total panel replacement (cheaper parts, but more labor).
Do not just install a 2 pole 50 amp Breaker. First your wire would need to be rated for the correct breaker. Second if the oven is rated at FLA (Full Load Amp) not to exceed 40 amps and there was a fault, the 50 amp breaker may not trip and could start a fire or kill someone. If you can wait look at Amazon, one quick search and there it was. Also look at a breaker broker they may have one, but be ready to put out was money, federal pacific are not cheap.
After reading your comment below, if your panel is now not allowing you to change out a breaker you have a bigger problem here. Your panel is starting to degrade and you need to call a electrician and get a quote to change the panel.
Safety over current devices are uually easy to replace and most makes are interchangeable.mcbs are often clip on a rail and electrically connect,oddball makes are on the market,seek advice if in doubt or get an electrician to do this.Be safe be sure.Steve uk engineer.Do not fit a higher rated mcb,like for like only. Beside amp rating,the type is important.See other answers as well.pn,make sure it is an appoved mcb.