The post card above is of a bridge over the Charente River in Civray, see if you can see any changes by looking at my photo taken in 2011. One obvious one is that there were no railings in 1924.
There are various sources of old postcards. There is a web site called Bid start and they have hundreds of postcards which you can buy for not too much. I did contact them to check if it was OK to feature a couple of their cards and to link to their site, but no reply, So I assumed they were not too bothered about a few of their cards being copied for this post. They do all show their logo CPAPHIL prominently in the middle of the card.
Other sources on the internet are Ebay and Moodys collectibles
Local markets also normally have one or two traders selling postcards. Beware copies, some traders use a high quality photocopy of the card, so what you are getting is not the genuine article. If it has a stamp with post mark on the back then you can be sure it’s not a copy. You may not be too concerned whether it is a copy or not. However if you want to scan the postcard and enlarge it to use it as a picture on your wall then you will get better results by working from the original.