It is FAR, FAR too early to look at any weather forecast for Florida for June. Anything beyond 4-5 days has little accuracy. Wait and look again when you get much closer. Rain is very common in Florida in June. However, it's not likely to rain all day...not that it can't, but it usually doesn't. It'll be sunny, then a nasty t-storm will come through, then the sun will come back out 30 minutes later. That's common, nearly every day. Or you may have plenty of days where they predict scattered t-storms and you get nothing. Last summer we were there for 17 days...I'd bet they predicted scattered storms for at least 7 or 8 of them, if not more. It rained on us exactly once. We had a few nights where it rained overnight, but who cares.
As for ponchos, yes, search harder at Wal-Mart. They're in the camping section. If your local store doesn't have them, they can have them shipped in for free. They're super cheap, really easy to use and store. Just personal opinion, but I wouldn't spend my money on a Disney one. I'd rather use that money for a much better souvenir or experience.