You would want an 8 speed ring, but it really doesn't matter.

You can go to a triple setup, putting three 110 mm bcd rings side by side; you'd have to get new chain ring bolts. Triple chain ring bolts are sold in sets of five, complete with spacers. So the longer bolts would go through your small ring, spacers, middle ring, crank arm spider, and large ring respectively. BUT you would also need a longer bottom bracket spindle and possibly a new front derailleur, all for very little benefit.

I'd suggest you count the teeth on your existing setup and enter them into Sheldon Brown's Gear Calculator:

Sheldon Brown's Bicycle Gear Calculator

then play with different options. As you'll see, your existing setup probably has a lot of redundant gears since there's little difference between the two chain rings. By putting a smaller chain ring on the inner position you will reduce that overlap and get some lower gears.

For simple purchases like one chain ring, the best selection is often ebay.