The typical method for planting a large area with lavender is to order trays of lavender plugs of the varieties that you are interested in establishing. Lavender seeds are tricky to get going; you have to keep the temperature at 70 degrees or better or they do not germinate (same for Rosemary). Here in the South I do get some volunteer lavender crosses from my English varieties but they ONLY appear in my gravel sidewalk (gravel retains the heat and encourages the sprouting), I've NEVER had a lavender spring up in the flower bed. Also, the larger lavender varieties (which it sounds like that is what you're interested in growing) are sterile hybrids....ie no seeds, all plants are started from cuttings.

I started my lavenders by buying a small plant of each variety that I was interested in trying. The next spring I took cuttings with a woody tip and started new plants. To me this was the most economical way to get a large number of plants: get cuttings of a variety that you like and start your own. Keep in mind that you won't have a hillside of lavender in bloom UNTIL your plants are two years old.

I am not familiar with your growing zone; but my favorite lavenders for "large spreading bushes" are Provence (silver-blue flowers, sweetest scent), Grosso (purple flowers and a strong camphor-lavender scent), and Provence White (white flowers, sweet lavender scent).

Provence - from my front yard
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Grosso - from my side yard
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White Provence (I borrowed this image from online, I don't have a good close-up picture of mine)
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White Provence in my veggie/herb garden, located to left of photo
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I work for a lavender farm and we plant all the lavenders in our zone six farm from plugs. Next year they will be from cuttings as we're looking to triple our number of bushes (We have over 2000 now on about 3 acres.) If you're wanting to smother a hillside with purple and wanting to do it from seed, I suggest using a mix of sages and nepata (catmint) with lavenders near the front. Perennial sage and nepata will spread and seed themselves and help extend the lavender's bloom time. (The lavs bloom in late spring/early summer.) It will also lend some biodiversity and guard against diseases. To bloom properly and be healthy the lavenders should be cut in the spring and after blooming. If you really want to do it right, you cut them again in mid/late summer.

If you're set on lavenders only, cuttings are the way to go. Only thing I'd be careful about is keeping the little guys weeded till they're strong enough to smother their competitors. you can do this by staking a square of landscape cloth around them.

Also know this: Some lavenders get very tall depending on variety and climate! Some take the heat better than others. For us in zone 6-7 Provence and Grosso are our hardiest. You may be fine with a spanish (Intermedia) lavender. They seem to like the heat a bit better, but they die back more in harsher winters. Whatever variety you choose make sure your soil is gravely and well drained. They HATE wet feet. We actually plant ours in large hills (Like you do squash) for added drainage.