I have bought exclusively from Baker Creek for 3 years now and have been very happy with them.

Botanical Interests has decent selection of heirloom varieties, nothing like BC or SSE but not too bad. I buy a lot from them because they have a good product and they are local (to me).

I'm not in your zone, but you might consider:

Fedco Seeds. They're in Waterville ME, which is 5a, but plenty of places in Maine are 3-4 (though folks like me from warmer areas buy from them also). They stock more than solely heirlooms, but I'm sure you'll find plenty to like on their list.
http://www.fedcoseeds.com/seeds.htm

Johnny's Selected Seed is in Winslow ME (also 5a). They're less into heirlooms, but still a good source.
http://www.johnnyseeds.com/

I'm in zone 5b in Missouri. Baker Creek is a little south of me in zone 6. I really like Sand Hill Preservation Center in Iowa. I don't know his zone but I suspect it a zone 4. He has all heirloom seeds. He doesn't do internet ordering. You need to send him a check in the mail. He's a little bit slow since he's a mom and pop company. He gives you great value for your money and has a good selection of varieties.

I also know of Southern Exposure Seed Exchange in Virginia but haven't ordered any thing from them.

You might consider Sandhill Preservations.

Dean

Here is a link that might be useful: Sandhill Preservations

I consider each of the following sources to be reputable sources of seeds and knowledge - I have gotten more reliable seed (true to description and germination) from all of these over Baker Creek over the years. Baker Creek has a great catalogue but the seeds have not lived up to hype too many times for me - lousy germination, not true to type. If I see something I am interested in - I source out from the places below:

Tatiana's Tomatobase Seed Shop - in BC - addicting with infoto learn/pics to drool over/common and rare heirloom seed (you've been warned)

Heritage Tomato Seed in CA - great guy, great seed - tomato porn pics - VSFW LOL

Sand Hill Preservation in IO - love what they are doing - they have all vegies and slips! Be patient with them

Gleckler Seedsmen - in OH - another family who is doing this in addition to their day jobs and families - awesome tomato seeds, pepper and a bit more

Mariannas Heirloom Seeds - in MI now (was in TN) - enthusiastic with awesome variety

Victory Seeds in OR - solid folk with good seed

Southern Exposure Seed Exchange - in VA Awesome dedicated folks with broad variety of all vegetables

The Sample Seeds Shop - in NY.Great variety of heirloom tomatoes, peppers, basils in smaller units; very reasonable and great germination.

Seeds from Italy - in KS Awesome source for Italian vegies - fantastic prices and germination

Tomatofest in CA - sort of like Vegas for heirloom tomatoes - glitzy and oh so good

Tomato Growers Supply - in FL (these are the folks who developed Black Cherry - no chit!) awesome for all members of nightshade family

My preferences:
For heirloom tomato: tomatofest.com; always good germination.
For various other seeds: sustainableseedco.com; good germination.
For some heirloom (they carry hybrids too so need to check plant by plant basis): mvseeds.com; good prices, but occasional inconsistency with seed quality (germination) and labeling mix ups expected.