Given we are both tall with 36" inside legs, we needed extra space to be comfortable in our early M3W. We made a spacer ourselves from a 50 mm long block of round aluminium bar of 3 inches/~75 mm diameter. This came from a brilliant website called Click Metal that cuts to size. (
http://www.clickmetal.co.uk) The metal was just over £5 + VAT + postage and was immediately usable without adjustment. Careful marking, drilling 3 holes with a pillar drill and buying suitably long bolts plus nyloc nuts and a spray of matt black paint meant the whole job was around £20....
We did look for a ready made spacer but could not find one that did not cost £50+ and require adapting - since we have the kit and enjoy tinkering, the DIY route was the way to go. Fuel capacity is now the limit for time at the wheel.
Usefully the spacer does not do enough for our 7 foot tall son to get in and drive which saves all the arguments about insurance for a 23yr old.....