I just bought some this year to try but what I found is that apparently there are 3 sizes. 36, 42, and 50 mm. I found the 36 mm kits in Walmart, HD, Lowes and the 50 marked as tomato pellets. There is a feed and supply store in my town that sells the pellets individually and they are only aware of the 36mm size. The packages of refills that I have seen online all look like the same packaging that I saw in Walmart and Lowes for refills which were the 36. Only Lowes has the refills for the tomato size. I read a review somewhere and someone else had the same problem as you, he had the 42 size kit and could only find the 36 refills.

Jiffy makes several sizes

Here is a link that might be useful: jiffy

Aquawise, I can't thank you enough for that link. Wish I had found it before ordering from eBay. Their prices are great even when you add in UPS shipping.

chele, add another size to that list-- as I wrote I have 300 30mm pellets in my possession.

I guess the previous batch I used last year must be the 36 if your sizes are correct. I thought they were plenty large for tomatoes-- certainly ended up with exactly the number of mature plants I was going for.

Thanks for that link! This is my first year starting seeds indoors so I picked up some of the pellets to try. I'm not sure if I like them or not but at least I have a source if I do.

The trays I have hold the 36mm and the 50 so at least the 30mm would fit in the 36 trays. I also got a bag of Jiffy and Miracle Gro seed starting mixes. I thought I'd try a little experiment. 3 days ago I planted lettuce using all 3 methods. So far, I am starting to see something in a few of the MG 6 packs, maybe 1 cell in the Jiffy mix pack and nothing in the 6 pellets. I put the 2 6 packs in the same tray with the pellets so they all got the same heat, light, etc.

I'm glad to hear they worked well for your tomatoes. I had picked up one of the 36 ct tomato trays which is larger than I need but then found a 16 count so I might try those if the ones I have now get some sprouts.

The 30s don't really fit well in the 36 holder.
I'll ask again since I may have buried this question-- any downside to just filling up a tray with the pellets basically all touching each other?

It seems the pellets don't work so well for some people-- my experience with them has really been just fine. Can I ask if you used hot water as per the instructions for the initial blow-out? How do you go about placing the seeds in the pellets?

I started having tiny sprouts out of the pellets yesterday. I don't see that it would matter having them in the tray touching. Once mine got wet, they didn't fit in their individual section in the tray anyway. They are sitting on top and touching each other. I think it is more of an issue once roots start growing. i used warm water from a pur filter. I just pulled back the covering, ripped it a little at the top and used a pencil to fluff it up like it said.