There weren't any 3.8 M5s officially imported into the US that is why it is hard to find them.
Unless you want to import it from Europe, you can but it has to
pass US emission standards.
Comparing the M5 and the M540i sport there is quite a difference in performance even though the 540i has a V8 engine.
The Canadian M540i has more special features of the M5 than the US version
Specs: E35 3.8ltr M5 (Euro)
3.8L, 340hp, 295lb-ft in-line 6
6-Speed Manual
Nurburgring & Adaptive-M suspension
18" M-Parallel Spoke wheels
345mm floating front rotors
///M5 badging and sill-plates
215 Saloons made, 0 imported to USA
119 Tourings made, 0 imported to the USA
1995 Canadian ///M540i
4.0L, 282hp, 295lb-ft V8
6-Speed Manual
Nurburgring & Adaptive-M suspension
18" M-Parallel Spoke wheels
345mm floating front rotors
///M540i badging and sill-plates
32 made, 0 imported to USA
1995 US 540i Sport
4.0L, 282hp, 295lb-ft V8
6-Speed Manual or Auto
Nurburgring & Adaptive-M suspension
17" M Turbine-II wheels
315mm front rotors
///M badging and sill-plates
200 made & imported to US
Info from Kindel.com
The U.S. 1995 540i M-Sport and the Canadian ///M540i are not really ///M5s. First of all the 200 540i Sports sold in the U.S. were all manufactured at the "normal" BMW plant at Dingolfing whereas all other ///M cars have been built at the M-Technic factory in Garching. These cars appear to be some of the last E34 cars off the Dingolfing line. The 32 ///M540is sold in Canada were (apparently) "hand-finished" at Garching next to the M5s.
In BMWNA's early 1995 literature the 540i with a 6-speed manual transmission was referred to as the "Sport model" because it had sport seats and an M-Technic (lowered, firmer) suspension. However, in the 1996 literature (published in late 1995) they refer to the 540i with the 6-speed as the "540i manual". The new limited edition car is called the "540i Sport" or "540i M-Sport" .
The 540i Sport package was an around $8000 on top of the regular 540i price. It was Option 812 on the 540i and 813 on the 540iA. The original sticker for my 540i Sport, the Sport option is listed as $7,600 (in addition to the $1,500 ASC+T traction control option).
more info go here ==>
http://www.kindel.com/BMW/bmwna_540i_sport.htm