I needed to order Tanzanian shillings. The only vendor in the UK is ManorFx (a trading name of Leftover Currency Limited).

• https://www.manorfx.com/

They require payment by bank transfer and my bank warned me (repeatedly) that they could not verify the vendor. I clicked through, made the payment and received my currency as promised.

You should of course do your own due diligence because things can change over time.

• Check out Trustpilot (good)
• Images of shop on their website (good)
• Google Streetview (good)
• Companies House (good)
• Linked-in (weak – no employees listed)

I will use them again. You should check the rate they offer carefully (exchanges rates for rare currencies are bad everywhere). I found that the Post Office was substantially cheaper for USD and I could collect it the same day.