You can always buy new seedlings when it comes time to increase your growing capacity, but you may want to use your existing plants if they are doing very well for you. If you have a very successful strain in your garden then it would pay to continue with the same plants rather than trying new ones (even those of the same variety).
Be aware that this is not a simple or quick process. You really only want to try these techniques if you have a particular strain of blueberry that is working out very well for you. In order to have new plants that produce fruit the same as the originals, you will want to propagate them with cuttings rather than trying to start new plants with seeds.
First you need to select the right pieces of your bush to use as cutting. Choose a vigorous and healthy plant that is showing no signs of disease. Use a sharp knife or pruning shears to cut away a new growing shoot from this year's growth. It should be no thicker than a pencil, and you should cut it after the plant has gone dormant for the winter. The piece should be at least 6 inches long and have several good leaf buds on it.
From the end that was cut from the bush, peel back a layer of bark for about an inch from the cut end.
You don't want to plant these cuttings in plain potting soil. The proper mix of materials will provide the acid environment that the blueberry prefers and it will encourage proper root growth. A loose 50-50 mixture of peat moss and either sand or vermiculite (a light flaky product sold in garden stores for potting plants) works the best.
Fill a small pot with this mixture and water enough to wet it down. Dip the end of the cutting piece (where the bark has been removed) in a commercial “rooting hormone”. This is a powder sold at any garden store that helps encourage root development from cuttings. Slide the cutting into the potting mixture, keeping it as upright as possible.
Water it regularly whenever it starts to dry out but do not soak it either. If it seems to be drying out too quickly, cover the cutting and the pot in a thin sheet of plastic to hold in the moisture. With some luck, the cutting will eventually put out roots of its own and a new plant is born.
Because you took the cut piece as it was going dormant, it can take a few weeks for the piece to “wake up”. Do not be discouraged by a lack of development in your cutting. If the buds start to open up and turn green, then you know the plant is thriving, though there may not be any roots yet. If nothing happens after about 2 months, you should pull back the peat moss and see if any roots have started. Take great care when doing this or you could end up breaking the new roots and ruining your cutting.
Not all cuttings will readily take to root, so it may take a few attempts.
Once you know your plant has grown its roots, you can carefully replant it into a larger pot with regular potting soil. You shouldn't try to remove the seedling from the peat moss, but rather just take a clump of the moss along with the seedling into the new pot.
You can replant your seedling directly outside but keeping it in a pot for a year will give it a better head start before being put outside. From there, you can treat it like any commercially purchased seedling.
Some varieties will propagate better than others, so you may want to try certain types of blueberries if you are new to this procedure. Patriot and Northland are very easy to root from cuttings, and Earliblue is known to be a little harder. Spartan and Bluecrop are both hard to reproduce this way, so you should plan on doing several before you get any of them to root successfully.