How can I type ティ "tei" (small ィ) in Japanese on OSX?

Just type texi in Katakana mode. Generally, prefixing x will generate the small variant character (useful if you need to generate ゅ, ょ or っ in isolation).

The x prefix does generate the small variant, but what you probably want is thi.

Building off of Justin's answer, there's a few other special katakana input strings that I found online. I added the "hiragana" versions as well based on the keyboard I'm using (Swype), but obviously not all of the input strings produce hiragana versions. Also, this is definitely not an exhaustive list.

wi = ウィ/うぃ

we = ウェ/うぃ

va = ヴァ/ヴぁ

vi = ヴィ/ヴぃ

vu = ヴ

ve = ヴェ/ヴぇ

vo = ヴォ/ヴぉ

she = シェ/しぇ

je = ジェ/じぇ

fa = ファ/ふぁ

fi = フィ/ふぃ

fe = フェ/ふぇ

fo = フォ/ふぉ

che = チェ/ちぇ