Hat pins or hard head pins are what you need. A good size for cards is about 3". If you want to leave the end sharp and have the pin removable from the card (so it can be worn as jewellery and therefore doubles up as a small gift) you can get protectors for the ends. Try the jewllery supplies section of your local craft store or eBay - I like the slim ones and they're usually called bullet clutches (this eBay seller has them
here so you can see what I'm talking about). They basically have a rubber stopper inside the metal so the pin stays really firmly gripped in there.
If you do them just on the card, you can rely on a ribbon or whatever it's pushed through to keep the beads in place but if you do them removable, you'll need to make sure the beads are secured on the pin. A crimp bead at the base of your arrangement is a good solution or you can glue them with your favorite jewllery adhesive.
ETA - I forgot the other thing I sometimes do to protect a sharp end on one that's intended to stay on the card. You can glue a bead on the sharp end - bugles are a good choice but you can also do a few seed beads.
Here's an example in my gallery with a bugle glued on the end - I coloured it first with a Krylon pen to make it look more metallic.
HTH, have fun!
Pretty pins on cards-how to make please?
