Digital clock source
My go to device for an adhoc clock is the hex Schmidt trigger device xx14, whether LS14, HC14, or AC14.

Ok, you've bought six in a pack, but the pack costs pennies, and you're not going to need a few extra inverters somewhere? ...as LED drivers? ... as input buffers?

The CMOS devices can go sub-Hz with big resistors, and with no explicit capacitor (only strays) and a &lt;1k feedback resistor, the AC device can crank out many 10s of MHz. How's that for flexibility?

However, 50 EUR/USD/GBP will get you a 60MHz function generator on xBay, with a nasty user interface, but it's functional. I've got one, it works, 'nuff said.