For trailing greenery in baskets there is ivy, bacopa, Lysimachia and nepeta, all different shades of green. They are all coming unto the GCs now, (here at least). Some greenery can set off the flowering plants to advantage.
Trailing verbena is pretty, too.

the petunias will trail down a bit when fully grown and flowering :-)

Our bedding annuals are only just coming onto the market stalls now, and this is considered quite early, even here where we generally get warmer weather earlier than you. We used to run a bedding plant sale (when we were living in the UK) to raise money for charity, and this usually took place from the beginning until about half-way through May. These days, when there are tiny plant plugs available, that might be considered a bit late, but the plants were quite big and well-formed, and no-one seemed to think it strange.
I am sure you will find bedding plants on sale already, even before the possibility of frosts is over.

You could try fuschia, my local flower market has started selling them now, still couldnt go out till prob mid - late may but they come in trailing and plenty of colours.

You should find plenty available in garden centres, supermarkets and at local markets, also on line. They are cheaper at the moment because they are small plants. Plant up your baskets but keep them in the greenhouse until about the end of May - after the last frosts. By that time the small plants will have filled out and your baskets should look as good as any you can buy ready planted at that time but for a fraction of the cost. harden them off by placing outside during the day for about two weeks before they go out for the summer. Any of the suggestions above are good choices.