Hello !
Go to a car graveyard and ask to see the ignition coils. They should have a large bunch of them from various car component makers. Bring with you your ohmmeter and choose the one with the closest primary resistance matching the OEM coil of your bike. If you have points it will be around 1.5 Ohm if I'm not mistaken.
If they are like here, they will force you to buy the set of two coils, but take them and you'd have a spare. Here, i pay 10 € for the set including the plug wire.
The most difficult part is to get a proper jig for installation on the bike... (you do not want to loose the coil during riding, isn't it ?)