Koko - is it your intention to show the sanded & stained floor. How old is the house? With very old boards (Victorian & Edwardian for example) you'll find it difficult get a match, off the shelf, with the size as more modern boards tend to be narrower (+t&g); you'll also not get a colour match. Modern house - then you are likely to get a good match. Matty's advice about getting the timber yard to cut to size is correct. You could, of course, obtain some reclaimed boards (easy to get).
Equalising the colour, with stain, over a floor containing a mix of boards is possible once the whole floor has been sanded. If you want a pine effect get a 2.5 or 5L can of Antique Pine stain (water or spirit based), dilute a small quantity, apply to the lightest boards to bring them near to the colour of the rest (you may have to do more than one coat). Don't be tempted to use neat stain at this point to rush the job, use a weak mix remembering the point is to gradually colour the lighter boards. Once you've evened out the colour, give the whole floor a coat of neat stain to equalise.
If you want to varnish the floor you can get this with stain pre-mixed but you'll still have to have done the above first.
Always try to get your timber in a 'proper' timber yard, they should know how to keep the stuff stored correctly, it'll always be better quality and will usually be cheaper/better value. Trouble is they usually aren't open on Saturday pm or Sundays.