Go to Wal-Mart. They don’t do it themselves anymore, you put it in an envelope and they send it off to Fuji in New York to have processed. The prices are very good, I suspect at or below cost. They will also do 120, 220, c-41 or e-6, just write the special instructions for what you want. They will also give you prints if you need them.
Another more online version is to buy prepaid mailer envelopes. Stick you film into the envelope, send it off, and they will come back in the mail when ready. You can buy them here.
Obviously there is no full online way to do this, since you are dealing with a physical medium.
You can trust the WalMart service, they send it out to professionals, so you don’t have to worry about the slacked jawed yokel behind the counter screwing it up.