I got this brazilian walnut wood. The tongue is like .225 and not the normal 1/4 inch and the manufacturer does not sell splines/slip tongue. How do I make my own spline? Can I buy a standard 1/4 inch one and then hand plane/power plane / sand it down?

Thanks a ton for your expert advice, guyz.

You have the right idea. It is better to modify a spline made with a shaper than to attempt making your own. I have done it both ways and I endorse the former.

I'd use a thickness planer to get them down to size. I got a Dewalt on sale at Sears and it has no trouble with IPE wood (which is probably what they are calling brazilian walnut).

Do splines have to be made from the same material?

I think which is better or easier depends on what tools you have. For example, I think it would be easier to plane a long uncut plank then to re-thinkness a bunch of small sticks.