Unfortunately, it's really hard to just "buy" a tackle with a draft pick. Teams have been reaching for OL more and more aggressively in recent years- in the past draft you had a couple guys with consensus 2nd round grades going in the top 20 picks. Even if you draft a tackle high, that's not much of a guarantee. Only a few tackles drafted high the last several years have lived up to the hype, Russell Okung being one of them.
I think FA or Trade should definitely be on the table, and it's pretty common to find guys who are close to league average tackles for relatively low cost during offseason free agency. Eugene Monroe was basically dealt for a song. There are Eugene Monroe type opportunities fairly often, so I think the veteran route makes the most sense, especially if the team believes in Bowie and Bailey to develop.
Also, sometimes very good tackles go with later picks. One of my absolute favorite tackles from the last draft was David Bahktiari, who went 4th round to the Packers. He's already become a very good player for them, to the surprise of all except for Ted Thompson and myself. Jordan Mills was a pretty good prospect too. He went mid rounds and has received strong reviews for his play this season.
But you can't bank on finding those hidden gems every year. I think that Seattle passed on both of them multiple times is a little alarming; in the case of Bahktiari especially I thought his talent was blindingly obvious, so I'm not sure we can hold out too much hope of Seattle identifying who those undervalued bargain buys are at OL.
Of course, the smart thing to do is to just go all out. Spend a mid-round pick. Spend a couple late round picks. Trade or sign for a veteran. Make tons of moves. Throw stuff at the wall and see what sticks. It worked for the secondary a couple years ago. It worked for the pass rush this year. It will probably work for the OL next year. Compete, compete, compete.