Go to the secret woods every day. It's south of your farm to where Marnie is, then straight west from there at the top of the map. You can chop the stumps there for 12 hardwood every day, they respawn each day. You will also occasionally get Mahogany seeds from them. Plant those in places where people won't walk all over them (like along the route from your house to Robin's and especially in the railroad area) to grow trees you can also chop down for hardwood. The process will also accelerate you moving towards foraging 10, at which point you can choose the Lumberjack profession to have a chance for hardwood from any tree.
The secret woods is the answer though; if you have 30 you can have enough to do the upgrade in just 6 days time. Hope that helps!