Kim - I'm sure you can, but I would strongly recommend buying your own to ensure they are of good quality, fit correctly and are BS approved.
You can pick them up pretty cheap in the UK at places like Aldi and at least you can be certain that you and above all your children are well and properly protected.
If the kids are in a school it's compulsory for certain ages.
There have been loads of discussions re this topic and most people suggest purchased before you go. They can be purchased cheaply and don't weigh much, just take a up a bit of room in your case.
We got new helmets in the sale in aldi last year £2.50 each!! I think they're about £20 to start with I worry about them getting banged in the case which could render them faulty but I wrap our polo necks and ski pants round them and put a pair of gloves inside them, just to give some extra padding
Hi Kim, we were in Borovets over Christmas and I hired helmets for my 2 children my youngest who is 9 had hers knicked on day one and they tried to charge me 50 levs for a new one! The kids were not made to wear helmets but I have now bought them both helmets from Aldi £14 each (£20 to hire each) as we are going back again in March!