A local farm supply/feed and seed etc that supplies market gardeners/small farmers is your inexpensive route. If you want more choices thatan the two three varieties normally carries, than good on line companies include Ronniger http://www.ronnigers.com/
Maine Potato Lady https://www.mainepotatolady.com/productcart/pc/home.asp Tucker Taters http://www.tuckertaters.com/ Moose Tubers http://www.fedcoseeds.com/moose.htm Woods Prairie http://www.woodprairie.com/
I would suspect that late March early Apil would be the appropriate planting time in your area, but suggest you talk to ag extension agent or a local grower.

Do you get any of the many seed catalogs? Most all the seed companies offer seed potatoes. And online there are lots of sources - Ronniger's is often mentioned and they offer lots of unusual varieties. If you Google 'seed potatoes' you'll find lots of sources.

And at proper planting time in your area most all the feed stores, farm stores, big box stores, and even grocery stores offer the common red and russet types for sale.

Mid to late April is a bit late. They are one of the first things planted - old rule of thumb is potatoes in the ground by St. Patrick's Day - even earlier if the ground can be worked. We usually plant ours the first decent weekend in early to mid-March.

Dave

When to start, depends on your local last frost date.
I would plant them about 2-3 weeks prior to that.

Also, to get a head start, GREEN your seeds.
Google "how to green potato seeds"

When potato seeds are properly greened, will have healthy greenish sprouts ,before you put them in the ground, not yellow pale. This way they will come up much quicker. If after they come up and by any chance you get a frost, then you MUST cover and protect them.