I am definitely not knocking a DIY person but a man has to recognize his own limitations. Sounds like you are short on tools my friend. If I were in your shoes I would consult with a cabinet shop or millwork shop and just have them make the boards for you. It would be cheaper than buying a planer, a shaper and a joiner......Unless you feel you may want these tools for later use. I personally keep some rough cut oak in my shop just for this reason.I also have the tool kit to do these chores also. This problem is not as uncommon as you think. The fact is you can put a floor down today and next year you can't get the same product( even from a large company that is still in business) Crazy but true. I always leave extra boards with my customers for that reason
The bright side to your problem is your floor is real wood so you can make them!
I would not buy the boards already precut like your dealer is saying. It sounds like he is not really trying gto help you, only to make a sale. Like I said earlier find a cabinet or millwork shop and ask them if they have the cured wood( of the same species) If it is really old you may have to find some reclaimed wood. Just make sure the boards you get are dry. Hope this offers you some insight!
Jack L.