I'm not sure of the dimensions of your cake but there are lots of things that can transport a cake safely and soundly. Since it was buttercream, Wendy's advice to keep it cold is the first rule to follow. If the buttercream is cold, then it won't stick to the plasticwrap.
If it's a regular round (8", 9", 10", 14") and not more than 4" high (I consider that a pretty tall cake) then let me suggest a round cookie tin, the kind for butter cookies during Christmas. Use the lid as the bottom so that your cake plate or board sits on it (don't forget to use one or two pieces of masking tape rolled over on itself to anchor the bottom. Then invert the cookie tin so that it fits like a dome over the cake. You can even tape the lid and the tin together so that you can carry it easier. It's rare, but you can also find square and rectangular tins, too, for similarly shaped cakes.
Fancier yet and beautiful, too, are hat boxes. They come pretty tall so that you can even put a church cakewalk cake into it. But the high sides may make it difficult to get the cake in in the first place. Another caveat is that they were not made for food transfer. So if you are getting paid, I'd advise against using a hat box.
For sheet cakes, nothing beats the boxes that sheet pans come in. I went begging at the restaurant supply store last week because I had three full sheet sized cakes to feed the masses for a friends wedding (I didn't have the time to do their dispaly cake). Since the boxes were designed to fit full sheet pans, I knew the cake was going to fit. In addition, it came with inside supports so that you could fold the top over and rest assured that it wasn't going to fall on the cake and mar the frosting. I measured and trimmed the cake board to fit perfectly so that there would be no sliding (the cakes had to be transported 35 miles and dropped off at two different rest stops before finally getting to the reception) and yet easy to take out by restaurant staff. At the end of the night, I saw the boxes being discarded by the restaurant staff. Not one trace of buttercream on the boxes.
Less fancy but does the trick are the unperforated basins for salad spinners. The can serve as a cake dome if they fit over the plate properly. A few pieces of tape may be necessary to keep the dome from sliding and injuring the cake.
And if nothing else, never underestimate the possibility of buying a pastry box from your nearby bakery. They might have their name or sticker on it, but you can always redecorate the box to cover it up and put your personal stamp on it. Some party supply warehouses also sell party cake supplies including boxes.
I have a collection of plastic cake tranpsorters given to me by various folks who wanted me to supply them with my practice cakes when I was still in cooking school. They were always returned cleaned and ready for the next practice round.
