On the market you will find all sizes and colors, but if you want you can also do an R2D2 costume yourself. It isn't hard at all. The materials you need are: 2 pieces of white poster paper, 8 orange pipe cleaners, glue gun & glue sticks, white paint, white school glue, craft foam, duct tape and your own shoes.
The first step refers to the preliminary sketches. You have to make rough measurements of how much poster board you need to wrap around yourself. Then, you have to make sketches of the front and the back of R2D2 on the poster board pieces using images from the internet. Using the sketches, you cut out craft foam to match and super glued it to the poster board.
The second step of making your own R2D2 costume refers to the fitting. If you aren't good with clothing fittings what you can do in order to fit this to yourself is to held the two pieces of poster board together and made it smaller until you think it is a comfortable fit. What you have to do next is to mark this on the sides and use hot glue to attach the two pieces of poster board. To reinforce this, you can use layers duct tape to hold the pieces together.
In the same way you freestyled the body, you also do with the straps. An easier way to do this would be to measure your shoulders and then add a couple more inches, depending on your size.
The third step, essential for the R2D2 costume is to make the helmet. To make the lid, you have to paper-mached a balloon that you size by holding it up to your head in the mirror while blowing it up.
To paper mache, you mix flour, water, and school glue. Cut strips of news paper, dip it in your concoction and place on the upper half of the balloon. Once it is dry, you trimm around any edges where it wasn't even.
Then, you have to use white house paint to paint the lid and let it dry for a day. After this, you have to hot-glue craft foam in the patterns you want.
Regarding the shoes, you have to attache strips of poster board to your own shoes using a double sided piece of duct tape. Then you have to decorate them as you want. This it will be truly a freestyle thing and won't follow any pattern of R2D2 costume, except for the orange pipe cleaners that were supposed to resemble wires.
Finally, you put all the pieces together and you will obtain a great R2D2 costume. So, with a little time and a little inventivity, you can have the costume you want. Have fun!