Depending on where you are in the country, these are the "transitions" or "adapters"
These are the most common ones...
14mm to14mm male to male
14mm to 18mm male to male
18mm to 18mm male  to male
14mm to 14mm female to female
14mm to 18mm female to female
18mm to 18mm female to female
18mm reducer
18mm expander
18mm to 18mm male to female
14mm to 14mm male to female
10mm to 10mm male to male
10mm to 14mm male to male
10mm to 10mm female to female
10mm expander
10mm reducer
We have more of them, just ask and we can get them....but these sell 10X more than any of the obscure ones...