Bulding a physical calculator from scratch
A good beginners platform would be arduino. For language you will need to know some C / C++

Hardware you could either use a plastic matrix keyboard or for more of a challenge use some micro switches on perfboard

Good places to purchase from are farnell, digikey, arrow, or RS to name a few (just google them they should be top of the results) Ebay and amazon can be good for small quantities

Here is an example project you could draw inspiration from
Arduino Calculator