Have you tried to start your own seedlings to have plants ready for planting when the frost has gone? Here are some tips from growing my own seedlings .
Start with a sifted seedling mix (I use my own compost that gets sifted before adding to the seed trays). Make sure the mix is damp, not too wet.
Some seeds, such as tomatoes, chilli and basil need under heat to germinate while the weather is still cold. For this I use an electric blanket which I cover with something to make sure it does not get wet from watering the seedling trays. The trays themselves I place onto some mellamine trays so that some water can be under the trays. The electric blanket is always set at its lowest setting.
Seeds such as lettuce, dill, spinach and onions do not need under heat but also benefit from the sheltered indoor environment.
Evenly space the seeds and cover with just enough seedling mix to cover the seeds. Then water gently without displacing or washing out the seeds.
Clearly mark each tray with what seeds it contains and date of sowing.
At night, cover with a clear plastic to keep moisture and warmth in. During the day, ensure that the trays receive good light and remain moist. If they do not get enough light, the seedlings are inclined to grow "long legs" in search of light and the resulting plant is never very strong. When the days start getting warmer, the seedlings can even be placed outside in a warm, sheltered area with enough light during the day and bring them back in by the late afternoon.
Seedlings are usually ready for transplanting - at least to a larger container - when they have their first true leaves ( the first two leaves visible after germination are the seed leaves. Wait for the next two leaves to emerge, as these are the true leaves). When the frost has gone, seedlings can be hardened by placing them outside for short periods of time every day to get them used to the outdoor environment. Transplanting can usually be done at 6-8 weeks.