Anything from the Cateye Rapid series. I have a
Rapid Mini and a
Rapid X3 -- though both have soared in price by a fair margin since i made my purchase a while ago
Alternatively -- I also have one of these which seems to be a cheaper cateye Rapid X1 knock off. Nowhere near the rated lumens specified but still quite bright. probably as bright as my rapid mini which i think is around 25lum -- price has gone up by almost £1 since i bought mine. Still great for the money!! the output is probably between 30-50lum
Alternate alternative, (of the alternative variant) I do have a Topeak redlite mega thats absolutely BNIB thats been collecting dust on my shelf for a while now that you can have for a small fee if you have enough space on your seat post for one. I ordered two by accident a while ago and just decided to keep the other as spare.
If neither of these suit your needs.... theres always the
Smart Luna R1 from planet X. I have two of these from when they were selling them without the mounting bracket for about £2 or £3 each. I bought them hoping they would be the older R1 version that had a pulsing mode for night/club rides but they dont.
::EDIT::
PlanetX currently have 20% off on clothing, lights & eyewear. use code PRAUTUMN1720. Brings the price of the R1 to £8 but you still need to pay P&P.
PlanetX wont ship for free unless you spend around £100 which is just absolute bullsh*ttery at its finest.