This information is correct as of the Afterbirth expansion.
To unlock the "little baggy" item and this achievement, you must locate and pick up 2 unique syringes. Since the requirements are fairly straightforward, I will simply help you identify and locate the items that contribute to this unlock. This game has TONS of wrinkles, so any information early on can be crucial to making the most of your runs!
The "little baggy" item (a shop item) drops a pill upon pick-up, allows the player to hold 2 pills, and turns all subsequent cards on your current run to pills. This is not a popular item due to the greater value of most cards compared to pills. Fun fact unrelated to the achievement: find 3 unique syringes and you will be awarded the "spun" transformation (it's pretty sweet & counts toward your 1001% achievement).
Quick info: Though not a guarantee, demon beggars very regularly pay out with syringe items. Demon beggars can be found in normal rooms, but are most commonly found in curse rooms (rooms with "teeth"), Devil Deal rooms, and arcades (take 5 pennies with you to new floors for a greater chance to spawn arcades). Find a Chariot card, Algiz rune, or any invincibility-enabler for more demon beggar plays.
The Virus (red syringe) - find this in item rooms, by playing demon beggars, or by defeating the Lust mini-boss. This is not a guaranteed boss drop. Lust looks like this:
Roid Rage (green syringe) - find this in item rooms, boss rooms, and by playing demon beggars.
Growth Hormones (purple syringe) - find this in boss rooms or by playing demon beggars.
Speed Ball (white syringe) - find this in item rooms, boss rooms, or by playing demon beggars.
Experimental Treatment (yellow syringe) - this is the only syringe that requires unlocking. Kill Mom's Heart 7 times for a chance to find this item in item rooms, boss rooms, or by playing demon beggars.
As far as I understand, the
Synthoil (grey syringe) item does not contribute to the "little baggy" unlock, but does contribute to the "spun" transformation.
The Afterbirth+ expansion (not yet released on Xbox One) makes available 2 unique syringes:
Adrenaline (item rooms)
Euthanasia (item and curse rooms and by playing demon beggars)
I have not yet gotten access to this expansion and cannot confirm whether these items contribute to this achievement.
This is not a tough achievement. Just keep playing and keep an eye out for the items listed above! Good luck!