**Tyvek envelopes are like the gold envelopes you can buy, but they are made of a plastic like paper called Tyvek. It is virtually indestructible and very light weight.
You can buy plain Tyvek envelopes almost anywhere that sells envelopes - Walmart, Walgreens, the office supply store, etc. The USPS also provides Priority Tyvek envelopes for free with Priority mail shipping. You can pick these up at the post office or order them online at usps.com.
These are not flat rate, so your shipping charge is based on where it is going and on the weight of your package. Unless you are mailing something really heavy, it is usually cheaper NOT to use the flat rate boxes, even for priority mail.