There are several suggestions:

- Make a little more wine than your secondary fermentor can hold, and
keep the extra amount in a sealed container in the fridge. Use it to
top up as you need to.
- Purchase a bottle or two of the same kind of wine and use to top up.
- Use water. Be careful as too much water will dilute your wine.
- Use marbles or clean stones to raise the level of the wine. Be
careful with regards to cleanliness.
- Use smaller containers.
- Use a CO2 source (such as vinegar and baking soda, or dry ice) to
create a blanket over the wine -- pour out the CO2 from the container,
but not the liquid!