There's a few companies making 40t and 42t replacement cogs for 10 speed cassettes. Here's one of them: OneUp Components US - 40/42T Sprocket + 16T

This is a much better option than putting a 20t or lower chain ring on, as the smaller rings up front increase chain tension, while larger ones in the rear give the same benefit of lower gear inches, but without the increased chain tension. Larger rear cogs do, however, put more stress on the cassette body and hub, so you'll definitely want a nice beefy rear hub, but a beefy rear hub is easier to manage than constantly snapping chains with a 17t chain ring.