start now: many sellers are putting their houses on the market now to sell this year. the risk of waiting too long is that your house gets bought by someone else. If you don't find something now, you'll know the market well by the time you're ready to purchase and move.
you also might be able to find something now/soon; make an offer with a far away close date and a pre-qual (different from a pre-approval) as proof of funds/financing and come to agreement. some sellers might appreciate knowing their house is in agreement and that they have the summer to stay there. If you were to do this, I recommend extending the inspection period as well as mortgage contingency until it's closer time to close.
the other option is to find something, close this spring and leaseback to the seller until you're ready to move out. you could leaseback at the amount of your monthly payments or whatever is fair. the struggle on this option will be to get owner occupied financing when you do intend to leaseback for a couple months.
but with a creative mortgage broker, it seems doable.
Good Luck with the process. Have fun with it ;)