The 1.4 came in 100/133 FSB vartiations. So, depending on your chip, you could use even a very old motherboard, even using PC100 memory. Any Athlon compatible motherboard at all should work. You might try to find a used ECS K7S5a, as those were quite popular and stable "back in the day," and they could use both SDRAM and DDR, anything from PC100/133/"1600"/"2100".
Btw, if you use a really old motherboard (like 3-4 years old) that can only do 100FSB, even if your Athlon is meant for 133FSB, it will still boot and run, only at 25% lower speed (1050 MHz).
Don't spend much money, though! That chip is pretty outdated. A Duron 1.8 (e.g.) will easily outclass it at $40.
Look on the for sale forum here. An old KX133a motherboard shouldn't be more than $20.