Going East towards Montréal==
From downtown, take number 86 OC transpo bus to Elmvale and keep walking in the direction the bus was facing when it dropped you off, in about 15mins you will be at the intersection of St-Laurent and Walkley, then cross Walkley, turn left, and keep walking for about another 25mins. You will get to an onramp that says 417 East Montréal. You might wait a while because traffic is limited to the onramp. If you walk about another 25mins, you'll go over a hill and find a perfect spot with plenty of room for cars to pull over. Enjoy, and may the Force be with you.
There's also a pretty decent spot outside Lee's station. Walk up to Lee's Ave, turn right, and walk past the apartment complexes to the Trans Canada on-ramp. There isn't a shoulder, but there is a paved area right at the entrance of the On-Ramp where cars can stop. Wait times are usually under 40 mins, although can be longer depending on the day.
Going west towards North Bay/Sudbury==
Take the 96 bus from the Rideau Centre from downtown Ottawa towards Kanata (a lot of buses from within Ottawa will meet up here, but will drop you off on the other side of the mall. Walk through the mall to the Mackenzie King Bridge entrance to catch the 96). You can try getting off at the stop beside the Eagleton Road on ramp for the 417 as it has a good shoulder, or keep going west a little on the bus until it stops at the Terry Fox Centre. Walk up to Terry Fox Drive - there are two on ramps there and one of them has a good shoulder to stand on.