If you are looking for something to use as centerpieces for a more laid back garden or country wedding my personal favorite is to use potted herbs. They look great, they smell great and they are more earth friendly than cut flowers.
If this is something you are going to DIY now is the time to get busy.Get an idea of the arrangement you want and then figure out how many plants you will need per table.
Even if your wedding isn’t for months do your shopping now.The garden centers and farmer’s markets are packed to the rafters with an amazing variety of herbs in the early spring. Grab your sweetie on the next pretty day and head on out to the biggest market or garden center in your area.
Look for small (2″ or3″ pots) herbs in a variety of colors, heights and textures. Thymes are nice for a low plant, sage is a pretty color with a large leaf and rosemary will add height. Chives and lavender will flower, so will chamomile. Take some time to play around with a different mix to see what you like.
If you are using clay pots, I have found the best place to get the really small ones is at the craft store. The are inexpensive and plentiful. For the most impact, get a variety of sizes and remember to get the saucers to go with them.
Once you get home, you can paint the pots or put them outside to weather naturally. To get a really weathered look, go head and pot your new baby herbs and then sink the pots in your garden or surround them with mulch. The clay will start to get that weathered patina that you see here. 
If you are going the galvanized pail route, check out the either the craft stores or the paint department of you local home improvement big box store for deals.
Yes I do realize that you will have to take care of these plant until your wedding. You never know, you may discover that you had a green thumb all along.
Although you can do live plants, this arrangement IS done only with clippings stuck into oasis. I have the design books for this series of centerpieces and in those books you are given the way to make it as seen. Sad they chose to do that when using the live plant would be just as easy – probably easier because you don’t have the cutting. Also, if you have a good local plant dealer they may be willing to start your plants for you at the right time so you don’t have to care for them ahead of time and they will be the right size!