This adapter is available from Think Auto (Mocal).
Part number FC8741 (M20 thread)
Part number FC874U (3/4" UNF thread)
Phone number 0208 2323525.
A version is also available from MDC or Chalky on eBay.
Prices
Including VAT and delivery to the UK it's going to cost about £70 for the Mocal and £65 for the copy (prices change).
There is an all plastic version being sold through some suppliers. It's al black so easy to recognise.
Do not buy this type under any circumstances. There have been cases of the thread that holds the oil filter
to the adapter actually cracking and the oil filter coming off. It happened to me and destroyed the engine.
If you see the plastic one for sale, please let me know.
M20 or 3/4" UNF
There are two options on this adapter, There was an M20 threaded version and a 3/4" UNF
threaded version. Metric threads are more likely to be found on filters for Japanese cars,
Peugeot and Renault. 3/4" UNF tend to be found on Fiat, Ford and Rover.
Thanks to Bernardo for confirming that Mocal still sell both types.
The oil filter you get on your smart from new is just a paper filter much like the standard air filter.
In my opinion this filter is not up to the job, especially on a turbo assisted engine.

This hunk of aluminium (shown in the middle) screws in place of the original cover
allowing you to use a normal screw on canister filter as used on most other cars.
Not only do they filter out smaller particles they also feature an anti-drain back
valve and a bypass valve. Plus you can buy them from Halfords etc.
These features stop all the oil draining back into the sump when the engine is
turned off and should the filter become totally full of particulates it will not
starve the engine of oil through lack of flow.
Fitting