I just order a ton (literally a ton) of compost every year and spread it in thick layer on top of the patch(es). With some plants I dig and fill a hole with it.

At least in czech republic there are places that make compost as a part of their business. It's not expensive (~$50 per cubic meter = slightly less than 1 ton), though the transport can be more expensive than the compost itself.

The biggest hurdle (for me) is that they just dump it in front of the house, take your money and say adios. I have a row house and there is no direct access to the garden, so it's either buckets or wheelbarrow. Realistically it's buckets as the wheelbarrow would be difficult to operate in the meandering narrow corridor and couple of stairs up and down. I don't mind the excercize though.

Of course one can buy bagged substrate, but that would get very costly quite quickly even with 50-100m2 of patches.