The Roku 2 & 3 still have analog outputs:
http://www.roku.com/products/compare
$50 for the cheapest one and they have a terrifically simple user interface.
You can also pick up first generation Rokus on ebay cheap, I know they will work well with an r.f. modulator because I've used mine on several old sets including a Predicta.
http://www.ebay.com/itm/Roku-TC2N100...item3cdf572f37
The Roku is still my favorite streaming device because of it's super easy to use menu, I have a TV and a Bluray player with streaming and the Roku is by far the easiest to navigate.