Lol it is easy for me, and try to explain as much simple as I can

:
There are few possibilites:
First possibility in Order
- Madara faked death with Izanagi with his own EMS and don't loose eye due to EMS being perfect stage of sharingan which don't loose light, something like that
- Madara implanted in himself Senjuu DNA and while some of his research/adventures he got his hands on Uchiha sharingans or he simply killed one due to knowing that he would have to give his eyes to someone in future.
- Madara awaken Rinnegan and then implant them in Nagato AND he could use Izanagi cause of some danger loosing one random Uchiha's sharingan in process being left with just one spare.(maybe Nagato attacked him using Rinnnengan without even knowing it.)
- Madara is left with one Sharingan implant.
Second Possibility in Order
- Madara killed some random Uchiha or just got his hands on one pair somehow betweeen leaving Konoha and comming back to fight Hashirama. Why would he do that ? because he did it as failsafe for himself in case of Battle going wrong and Madara dying from Hashirama's Hand. Yeah it could be called anticipation.
- Madara used one of Random Eyes somehow activating Izanagi to escape death from Hashirama
- Madara implants Senjuu DNA and later awake Rinnengan
- Madara implants Rinnengan in Nagato and retrun with just one spare Sharingan as he used other one for Izanagi
Last Possibility in Order
- Madara didn't use Izanagi and used other technique or some other hax to surivives
- Madara implants Senjuu DNA
- Madara kill or not to get new spare sharingan's pair or just not yet
- Madara got Rinnengan and kill Random Uchiha to get his eyes or not anticipating fact that he will implant his own eyes in Nagato, then Madara uses Izanagi to escape to come back to cave.
- Madara is left with just one spare Sharingan
I am sure much more Possibilites are possible

, but I can't think of any more besides I won't waste more time for coming with new crazy ideas
Anyway for sure,
Key factors in that battle were:
1) If Madara used Izanagi or no
2) If yes the he must killed some Random Uchiha or get some Random Sharingan's pair or he used EMS with it and not loose light somehow else he didn't use Izanagi
3) Anyway he must killed or somehow get his hands of fresh spare pair of sharingan

, Whatevevr it was before battle with Hashirama and after leaving Konoha, or after implanting Senjuu DNA and waiting for Rinnengan or even after awaking Rinnengan(small possibility) but for sure not after implainting his own eyes in Nagato, because blind Madara couldn't even surivives let's alone get Sharingan somehow.
4) Did he do everythinh by himself, or did he use some other person help ?, maybe used Zetsu
That all I can come up with, I believe these above stated by Me Factors could be keys to solve your and our question
I hope I helped even just a bit