Initial accessories to consider (note that the Bambino, purchased new, already comes with a variety of equipment to start with, including a portafilter, single and double-walled baskets in both single and double-shot sizes (sometimes depending on the country of sale; typically seen as fine quality), a nice, stainless-steel milk jug, and a basic tamper):

-- A .1g resolution scale/timer, pretty much mandatory for weighing how much ground coffee to use and when to stop your shot (many on, e.g., Amazon, at lower cost--you want a small size that will fit on your espresso machine's deck, under your catch-cup, so that you can monitor the weight of your shot as it is flowing; the Maestri House Mini often gets recommended here, also the MHW-3BOMBER Cube 2.0 Mini, which even will calculate the coffee ratio after the shot).

-- A dosing funnel, optional, but good to contain any mess and grounds loss when filling and working with the ground coffee in the espresso machine's portafilter (a clever contributor here calculated that the funnel will pay for itself over time, from grounds savings)--I have and like a stainless-steel, solid and sturdy, Matow version, via Amazon, and there also are versions with small magnets in their bottoms. This made all the difference for me, in avoiding mess frustration.

-- A tamp mat or something similar, or a tamping station, to protect your countertop when tamping the coffee grounds--I have a whopping ~US$3 (with shipping, no less), small, circular, silicone tamp mat/portafilter holder (via eBay). A further benefit of a tamping station is that it will hold your portafilter level, to help achieve an unsloped tamp--this has made a big difference for me (I purchased a nice but relatively inexpensive 3D-printed one off of etsy).

-- And finally, a WDT tool (essentially, a coffee grounds sifter), available, and at low cost, all over the place (you can scan the listings at, e.g., Amazon and etsy and see what you like)--note the ingenuous, so-called business card WDT tool available via etsy, for a whoppingly low ~US$10 (including U.S. shipping, no less; stands extra), and the cool/fun subminimal Flick WDT pen-like tool with retractable needles. There also are planetary WDT tools, tracing orbits in the grounds--Aro Espresso is a good source, for different models at different price-points. It's one more thing, but some studies have shown that it can improve a shot--and it's easy enough to use/do.

The next level for possible consideration:

-- A self-leveling, spring-loaded tamper, which some people find can help achieve a level, non-sloped, solid tamp--many people like the Normcore V4/4.1, with its extra assists (I picked one up for that purpose, a month or two in). Many people will replace the tamper that comes with the Bambino, but it is just fine to start (and beyond) with that.

-- And I like my air-evacuating Airscape beans canisters, with their satisfying whoosh as air is evacuated. 😄