I'm going to be in Portland for a month, on business. Can any of you recommend a Salvation Army or similar establishment that i'd be able to pick up a bike for <$100 ?

I just need a beater to cruise around the city. I don't want to have to worry about a rented bike getting jacked or stripped.

Thanks for any recommendations you may have!

Well, there's several Goodwill's (including a huge superstore close-in southeast side) and similar thrift stores. But for a decent ride you'd probably be better off up'ing the budget just a bit and searching craigslist and/or a handful of places that specifically sell used bikes (e.g.; Recyclery, City Bikes, Sellwood Cycles, Community Cycling Center). You can return the bike for consignment or sell on craigslist at the end of your stay.

There's dozens of bikes listed on craigslist here each day.
https://portland.craigslist.org/bik/

I'd try the community cycling center https://www.communitycyclingcenter.org/ or City Bikes https://www.citybikes.coop/