There is an iTunes podcast channel, which you can add here: Roku iTunes Podcast Channel. I've used this on occasion, and it works well for me. It's free and doesn't require you to actually have iTunes.
You could also try this, if iTunes doesn't have what you need: Roku Nowhere Cast Channel. I haven't used this one personally, but the developer is one of the more prolific Roku developers. This page says it's a paid app, but doesn't list the cost, so you'll probably want to do some more research before using it, to make sure it meets your needs.