I've recently went through the same search my my 1935. Ideally, I wanted to get a thumb (micro) keyboard that snaps onto the bottom of the unit.
The problem with the 19XX series isn't the connector - the 19XX series just doesn't support serial. I guess they had to cut corners somewhere to get the price point so low. It's not a big deal really - unless you want to connect a keyboard.
The suggestion to use a IR keyboard earlier in this thread is, in fact, the only hardware solution available. The only other option is a software based data entry method. There are many available out there like Calligrapher, TenGo, Fitaly etc. (I personally like TenGo the best)
The one that comes the closest to emulating an actual full sized keyboard, in my opinion, is SPB Full Screen Keyboard: (I use this as well)
http://www.spbsoftwarehouse.com/products/fsk/index.html