There are two types of sourcecs for Little Bluestem or any other prairie plant. One is to collect seed yourself, the other is to buy seed. Collection is actually pretty easy this time of the year if you know of some native Little Bluestem in your area. I think it shouldn't be hard to find, since it grows not only on prairies but also on roadsides, fields, etc. Collecting seed will not impact the stand you're collecting from, since the great majority of the seed goes to waste each year, and you won't be collecting it all anyway. The standing stems will look fuzzy due to the feathery parts of the seeds sticking out. Simply grasp the stem and strip the seeds off. These should sprout for you in the spring if you handle them properly (expose them to cold to simulate winter, known as stratifying the seed). To buy seed or plugs, try one of the reputable prairie seed sources. Prairie Nursery and Prairie Moon Nursery come to mind, but there are probably a couple of others. A nursery will definitely be easier, but more expensive. Locally collected seed has the advantage of being local, but may be hard to grow into decent plugs in a year. Good Luck.

Where is the restoration in IL?

Here is a link that might be useful: Illinois Native Plant Sources

I've linked a wholesale distributor of plugs. Even if you can't buy from them you might find their list useful for availability and prices and minimum orders.
Plugs of warm season grasses should be planted before Labor Day. If you're on a limited budget, go with seed.

Here is a link that might be useful: North Creek Nursery

I am growing plugs of native grasses from seed for meadow gardens I am creating and to intermix in a small meadow area. These are very easy to start from seed. Using the winter-sowing technique, I sowed the seed in small hunks in 2 liter soda bottles. Grew a nice little bushy clump, which can be planted outright or divided into smaller plugs.

I bought the seed from Prairie Nursery last winter. Could not believe the quantity of seed! Thousands of seeds! There is no way I can use it all and would be happy to share some with you for a SASBE. (sasbe = self-addressed stamped bubble envelop) Send me an email if interested.

The native prairie grasses are warm season grasses, so they won't sprout until it warms up in the Spring. My experience was that they did not need cold stratification. I sowed Little Bluestem, Big Bluestem, Sorghastrum nutans, and Boutelouea curtipendula on April 12th. All 4 species sprouted within 10-12 days. We had some nice warm Spring weather last April, which helped the seeds to sprout quickly.

Here is a pic of the prairie grasses. They are growing in the bottoms of 2 liter soda bottles, which I cut off and removed the tops once after last frost.

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When you bought the seeds from Prairie Nursery, they were already cold stratified, ready for planting.

If you had harvested them, yourself, you would need to stratify.