Birdshot will do for most practice as a substitute for buckshot. Once a gun establishes what it will do pattern wise with a given buckshot load the main question has been answered, and practice with buckshot versus birdshot usually isn't necessary IMHO. Look at the ammo requirements for shotgun training classes by better known instructors for an example. (Louis Awerbuck
Tactical Shotgun - Stage I Ammunition requirement is 400 Birdshot, 50 Buckshot and 70 Slugs.)
Slugs... well, not much of a substitute out there for them. Just have to pay the bill to practice with the real thing. I find that Brenneke KOs work well in many of my working guns, and they can be had on sale at very reasonable prices from a number of outlets from time to time. Plus I practice with what I use for real, IMO a significant advantage where POI (point of impact) can change with various brands/loads/lot numbers as it can with slugs.
When I find shotgun fodder that works well, I usually buy it by the case. Sometimes that rates a discount too...
hth,
lpl/nc