You should find most mainstream manufacturers cars from the last few years fitted with ISOFix now, at least on the back seat. Some won't have the top tether anchorage (incl. the Leon 1) so you rely on the bottom 2 anchorages (only an issue with forward facing seats for older children)
The main
problem which has delayed ISOFix is that manufacturers started fitting it before the standard had been completed in Geneva, when it was completed and agreed it only applied to UN-ECE approvals which are not necessarily the same as those used for EC Whole Vehicle Type Approval.
Until the texts are aligned (they may have been by now) the manufacturer can choose whether to use the UN-ECE text including ISOFix or the ECWVTA text which doesn't require it.
You should never use ISOFix and the adult belt together on a child seat - the anchorage locations can be quite different between the 2 on different vehicles.
Adrian