Have a look around watchworx, they have some great looking, sensible priced leather straps. I've bought from there before and was happy with the straps I got.
I suspect your Seiko will look good on a black strap with contrast stitching, and there's a few to choose from. They also sell deployants you can swap the buckle for.
If you wanted to investigate rubber I'd recommend actual rubber over silicon, as I find it softer and more comfortable. My favourite is Bonetto Cinturini, and I buy from a place in Poland (here I think, curiously by workplace filter won't let me access it to check!). They have a good selection, the postage is a factor, but if you order a couple of straps it works out good value. Some of the rubber straps come on deployants too.