Words that are often written in katakana but have kanji available tend to fall into several groups. These are the sorts of words I find when I look for words meeting these criteria in Jim Breen's EDICT and such:
Recent loans from Chinese languages, reflecting modern pronunciation:
- Food words: ザーサイ(搾菜)・ワンタン(雲呑)・マーボー(麻婆)豆腐
- Numbers: イー(一)・アル(二)・サン(三)・スー(四)
- Mahjong words: リーチ(立直)・テンパイ(聴牌)
- Place names: ペキン(北京)
- Interjections: ニーハオ(你好)・シエシエ(謝々)
Of course, there's other sorts of loan words: クーニャン(姑娘)・ピンイン(拼音)・カンフー(功夫)
Older units given ateji, now often written in kana: メートル(米)・トン(屯)・グラム(瓦)・ページ(頁)
Foreign place names given ateji, now usually written in kana: アメリカ(亜米利加)・イギリス(英吉利)・ロシア(露西亜)・シンガポール（新嘉坡）
Some morphemes made by abbreviating these ateji are still used. For example, in the word 英語, the first part 英 comes fromg 英吉利.
Other foods given ateji: コーヒー(珈琲)・レモン(檸檬)
Plants and animals which are commonly written in katakana but have kanji available: ハチドリ（蜂鳥）・ホトトギス(杜鵑).
You'll run across some others here and there. Overall, out of the words listed in EDICT in katakana, around 1% have kanji. But there are a lot of rare ateji for foreign words that are normally written in katakana. Just take a look at this list, for example.
Of course, you should take these numbers with a grain of salt. The category of "katakana words" isn't at all well defined, and I'm certain there are many examples of ateji that aren't listed in EDICT. And after all, people can make new ones any time they want—like 吐露非狩古鬱, for example.
But most of the time, you should expect "katakana words" to be written in katakana.