There are Unlimited tournaments, but they're pretty rare.
Best thing to do in Modified is to create cheaper decks and borrow staples from people. The best cards that you want these days are:
4 Holon Transceiver (~$10 each)
4 Holon Mentor (~$4-6 each)
2 Holon Adventurer (~$.50-1.50 each)
2 Holon Scientist (~$.50-1 each)
1 Holon Farmer (~.50-1 each)
1 Holon Lass (~.50 each)
Then Holon's Castforms are pretty big right now, so you might want to get some of those:
4 Holon Castform ($4-8)
All prices are by eBay btw.
Those should get you started in Modified, then you can get most everything else to make up your deck pretty easily and cheap, or just borrow it.
Also if you don't want to make the transition and just stay in Unlimited, try and develop a counter to Vileplume-ex, since it's arguably the best deck in Unlimited.
Arcanine out.