Scan eBay, craigslist, the Facebook LS pages, and junkyards. Individual parts are hard to find and a "kit" is out of the question.
All-red tail lights go for $150-400/set, ranging from "LS lights" to "I know what an LSE is"
Price for the front bumper: $100-$2000, ranging from "This is a Town Car bumper" to "I know what an LSE is and I'm a jerk"
Skirts go for probably $100-200.
The grille is usually up around $250
The Gen 2 LSE spoiler can go for $100-200
The bumper is the most defining characteristic of the LSE and has the biggest impact on changing the look of your LS. The skirts look great, but the car looks fine with or without them. Beyond that, I would consider "knock-off" options. The chrome surrounding the grille can be painted, and the grille teeth can have chrome strips applied to it (there's a kit on eBay). The two-tone tail lights can be wrapped in red vinyl for the cost of once trunk-mounted tail light panel. The "M3 style" lip spoilers go for $40 and have practically the same effect as the Gen2 LSE spoiler. Only a handful of enthusiast can tell the difference, and only within 20ft