Playmobil does not sale transformers any more. You can buy used transformers e.g. in Ebay.
But it's only in Germany very cheap, I think.
You can buy another transformer of UK (LGB-, Piko-, H0 transformer) which has max. 16 V DC (or don't drive full speed).
How many locos do you need?
If you need 3 or more locos, you need a transformer with more power (more amp/ampere), 2 A (amp) or more.
e.g. Piko 35006 has 1,6 amp, Piko 35002 has 5 amp.
The track connector (to the transformer) could also be of LGB (50160) or Piko (35270) - contacts better than Playmobil connector (4351 or 4372)!