SP101 night sights.
I out a Mepro tritium font sight on my .327 101. Easy to install; just had to remove the stock, drill out the new one and pin it in place. I still use the stock rear sight, but I'm not sure you can change that one on most SP101's.
I got mine from Midway for around $30, if I remember correctly. It's been serving me well for about 3 years now. I wouldn't hesitate to use another.