HGSS brings Gen 4's cross-generation evolutions to the Johto experience. Pokemon like Tangrowth, Yanmega, and Mamoswine are available through move-based and location-based evolution. Leafeon and Glaceon join Eevee's evolution family for the first time in a Johto game.
All 29 are available, but many require postgame access to Kanto. Tangrowth, Yanmega, and Mamoswine need AncientPower via the Move Reminder. Weavile needs a Razor Claw at night. Honchkrow and Mismagius need Dusk Stones. You can get some during the Johto story, but the full set opens up after reaching Kanto.
Leafeon needs the mossy rock near Eterna Forest and Glaceon needs the icy rock, but those are in Sinnoh, not Johto. HGSS works around this: trade from a Gen 4 game, or level up Eevee near the Ilex Forest moss rock (Leafeon) and the Ice Path (Glaceon) using special conditions. The exact locations match Johto landmarks that serve the same purpose as Sinnoh's rocks.
The Pokewalker (pedometer accessory) earns experience and can increase friendship for the Pokemon stored inside. Walking enough steps boosts friendship, which can help trigger friendship evolutions faster. You can also find evolution items on certain Pokewalker routes, like stones from specific course rewards.
Bill's grandfather on Route 25 gives you stones for showing him specific Pokemon. Bug-Catching Contest prizes include Sun Stones. The Pokeathlon Dome shop sells all stones for Athlete Points on specific days of the week. That shop is the most reliable repeatable source once you have enough points.
Technically yes, but it's slow. Friendship builds from walking, leveling, and grooming at the Goldenrod haircut shop. Before Goldenrod, your options are limited to just walking and fighting. Crobat is one of the most popular early friendship evolutions since Zubat is everywhere in caves from the start.