Antique dealers. I have got a lot of rules from "antique warehouse" or large open air "antiques fairs". In both of these cases you get several dealers with different ranges of interests.  Provided you have an idea of what is a reasonable price you do not need to pay an exorbitant amount - though some caution is called for as one dealer I have used had reasonable prices for top end rules but ridiculous prices for some cheap rules.

Car boot sales, flea markets etc. These are sales where you are dealing directly with the seller. Prices tend to be lower than through dealers but my experience has been that a lot of leg work is required.

Auctions. This is probably the best way to get rare or antique rules. Prices at sales where a lot of rules are offered, and competition comes from other collectors,  tend to be high. On the other hand at sales where there may be the occasional rule prices tend to be lower.

Slide rule trade fairs. I only know of one - listed below.