Edwardian theatrical postcards were produced in such numbers that they are still readily available to collectors today. When their popularity waned many thousands of theatrical subject cards were left as unsold stock, with the result that postally unused cards are as common today as used ones. Whilst used cards are more interesting for their message content, unused cards are often the ones in best condition. You can sometimes find cards in antique shops or old book stores etc., or you can bid for them in regular listings on Ebay. But by far the best way to obtain them is to go along to Postcard Fairs.
The Fair I regularly attend in Leeds takes place on the first monday of alternate months and regularly has stalls for around thirty traders. Some traders have a larger theatrical section than others and cards may be offered at very different prices. It is not unusual to see an identical card to the one you just bought offered for half the price on the very next stall. So it is advisable to take time to wander around all the various stalls to check out the prices before you begin to make your purchases. Of course, if you attend a particular fair regularly you will get to know who are the best dealers for your subject and henceforth use them as your starting point.
Regular postcard fairs are organised all around the country and a good source of information on these is the Postcard Traders Association website. For more information click on the links below.