There seems to be quite a market for old bus signs, but the prices are often rather on the high side. However Eden and Eden have a small selection of old British bus signs reasonable prices.
There is a choice of two numbers, 460 and 611 (I’ve no idea where these services ran) and a Private sign. They cost $38 for the number signs and $68 for the Private sign and are supplied framed. Eden and Eden is based in San Francisco, which is good if you in that area, but if you in UK, it may seem strange to be buying British signs in the USA.
Visit the Eden and Eden website for further details.