There are a few Pitts S2s available for rental - G-ODDS at Waltham for example (heir to G-STUA, possibly the hardest working airframe in British aerobatics in its day...).
From an outright ownership or share perspective, something on an LAA Permit is definately the way to go at Standard and Intermediate - Pitts S1, Laser, Extra 230, One Design etc. Our local Extra 230 took the Advanced Championship for the second time in a row this year, but the mainstream fare at that level and above have 6-pots and carbon wings, not until recently something the PFA/LAA would deal with, but I understand that an Edge-540 on an LAA Permit is on the cards. The Laser is a fine alternative to the Extra 230, but has on occasions shown an unexplained tendency for the oil filler cap and dipstick to come loose while performing inverted checks in the box at National championships, covering the aircraft and pilot in oil and embarrassment, and such events can take literally years to live down.
Regardless, the route to competition success lies through combining Precision with Clarity, and in order to do that you need to practice. A lot; and in combination with practice is the requirement for critique from the ground - so your best bet would be to find an share at a price where there is money left over for a significant amount of practice time, in an environment where there are a significant number of other competition pilots -at all levels - willing to reciprocate with ground critique.
Anyway....welcome to the Dark Side - where the term 'autorotation' has an entirely more positive connotation (generally).