Where Might I Find Tiny RFID Tags?

Unfortunately, I don't believe there is an authoritative source on miniaturization.

Hitachi's µ-chip (pronounced "mu-chip"), which measures 0.15 millimeter by 0.15 millimeter (0.006 inch by 0.006 inch), is the smallest chip out there (see Hitachi Unveils Smallest RFID Chip, Hitachi Unveils Integrated RFID Tag, Hitachi Shrinks Smallest RFID Chip and When Will RFID-Embedded Paper Be Available?). Most other RFID chips utilize external antennas to achieve a longer read range.

Hitachi Chemical markets the Ultra-Small Package tag, an EPC Gen 2 passive ultrahigh-frequency (UHF) tag measuring 2.5 millimeters (0.098 inch) square and 0.3 millimeter (0.012 inch) thick (see Hitachi Chemical Markets Tiny UHF Tag), the read range of which can be boosted by means of an external antenna or other metallic object. The tag consists of an Impinj Monza 5 chip and an antenna embedded in epoxy resin, and is designed to be durable enough to be applied via injection molding or incorporated into printed circuit boards.

In addition, Murata Manufacturing Co. offers what it calls the world's smallest high-frequency (HF) tag. Measuring 3.2 millimeters (0.13 inch) wide by 0.7 millimeter (0.03 inch) thick, the tag—model LXMS33HCNG-134—is one-tenth the size of most other HF tags on the market (see Murata Mass-Produces World's Smallest HF Tag), and can be attached to a range of objects, enabling item-level tracking of goods via a Near Field Communication (NFC)-enabled phone or other reader compliant with the ISO 15693 standard.

There are also tags with external antennas the size of a postage stamp (see RFID News Roundup: Avery Dennison Intros EPC Gen 2 Inlay for Tagging Small Items), while Omni-ID has created some small on-metal tags providing good performance. But shrinking a chip has not really been a big industry focus.