Anna, you could use some pictures of every day stuff like trees, dogs, cars, clouds, trains, fruit etc (just google them and print them or cut them out of magazines) also scan/print photos of family members - even the ones they don't see very often because you can tell bubs all about who they are. Also stuff like wild animals, aero-planes whatever tickes your fancy. Just so long as it's one picture to one card IYKWIM.
You can just use some craft glue and some cheap carboard (you could even just re-use cardboard boxes instead of throwing them out) to stick your pictures on. You don't need to make them all whizz-bang and flash because babies like to chew them and chuck them around and if you haven't put too much effort or money in you won't mind when they're ruined.
I think at 6 months what they like best about flash cards is your explanations/the interaction with you. Now Yasin is older he asks more questions about what is on them and he really enjoys animal ones because we make all the aninmal noises together.