Where can you get smaller envelopes?

Depends on where you are I suppose - but a stationery store should be able to accommodate you I would think. Even Walmart has different sizes of envelopes that you buy in packages. Come to think of it the local grocery store probably would as well...just not as big of a selection. Some place like Office Depot, or Staples would have a huge selection.

Here's an even simpler approach: why not include a regular-sized envelope and fold it in half?